Choosing reference genes for qPCR normalisation - Revision history

Philip Zimmermann at 19:25, 5 October 2011

2011-10-05T19:25:43Z

←Older revision		Revision as of 19:25, 5 October 2011
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	* Panels of "housekeeping genes"		* Panels of "housekeeping genes"
-	:All genes used for normalization can show problems in one or the other condition. There are always conditions in which their expression differs significantly from their general level of expression. That is why reference genes should be validated for one's condition of interest. Ideally, one should choose from the complete genome rather than from a gene panel. Genevestigator RefGenes [http://www.refgenes.org] is ~~a publicly available~~ online tool that uses a very large microarray database to identify genes that are most stable in conditions similar to that of your own experiment.	+	:All genes used for normalization can show problems in one or the other condition. There are always conditions in which their expression differs significantly from their general level of expression. That is why reference genes should be validated for one's condition of interest. Ideally, one should choose from the complete genome rather than from a gene panel. Genevestigator RefGenes [http://www.refgenes.org] is an open access online tool that uses a very large microarray database to identify genes that are most stable in conditions similar to that of your own experiment.

	== Reference genes across tissues ==		== Reference genes across tissues ==

Philip Zimmermann at 09:27, 4 October 2011

2011-10-04T09:27:49Z

←Older revision		Revision as of 09:27, 4 October 2011
Line 27:		Line 27:
	:'''[[User:Ajeffs\|Ajeffs]]''' 06:55, 21 April 2007 (EDT): 18S is generally a terrible choice for a reference gene thanks to the combination of (i) high abundance (creating a 1:100 dilution of template to run in parallel with neat template just for 18S is a complete drag); and (ii) having different degradation characteristics to mRNAs (it appears to be more resistant to degradation). However, if you can show that you have screened 5-10 reference genes, and 18S is still the best for your specific situation then so be it (but do try 28S if you or you PI is hung-up on 18S).		:'''[[User:Ajeffs\|Ajeffs]]''' 06:55, 21 April 2007 (EDT): 18S is generally a terrible choice for a reference gene thanks to the combination of (i) high abundance (creating a 1:100 dilution of template to run in parallel with neat template just for 18S is a complete drag); and (ii) having different degradation characteristics to mRNAs (it appears to be more resistant to degradation). However, if you can show that you have screened 5-10 reference genes, and 18S is still the best for your specific situation then so be it (but do try 28S if you or you PI is hung-up on 18S).

-	* ~~Reference gene panels~~	+	* Panels of "housekeeping genes"
	:All genes used for normalization can show problems in one or the other condition. There are always conditions in which their expression differs significantly from their general level of expression. That is why reference genes should be validated for one's condition of interest. Ideally, one should choose from the complete genome rather than from a gene panel. Genevestigator RefGenes [http://www.refgenes.org] is a publicly available online tool that uses a very large microarray database to identify genes that are most stable in conditions similar to that of your own experiment.		:All genes used for normalization can show problems in one or the other condition. There are always conditions in which their expression differs significantly from their general level of expression. That is why reference genes should be validated for one's condition of interest. Ideally, one should choose from the complete genome rather than from a gene panel. Genevestigator RefGenes [http://www.refgenes.org] is a publicly available online tool that uses a very large microarray database to identify genes that are most stable in conditions similar to that of your own experiment.

Philip Zimmermann at 09:27, 4 October 2011

2011-10-04T09:27:16Z

←Older revision		Revision as of 09:27, 4 October 2011
Line 28:		Line 28:

	* Reference gene panels		* Reference gene panels
-	:All genes used for normalization can show problems in one or the other condition. There are always conditions in which their expression differs significantly from their general level of expression. That is why reference genes should be validated for one's condition of interest. Ideally, one should choose from the complete genome rather than from a gene panel. Genevestigator RefGenes [http://www.~~genevestigator~~.~~com~~] is a publicly available online tool that uses a very large microarray database to identify genes that are most stable in conditions similar to that of your own experiment.	+	:All genes used for normalization can show problems in one or the other condition. There are always conditions in which their expression differs significantly from their general level of expression. That is why reference genes should be validated for one's condition of interest. Ideally, one should choose from the complete genome rather than from a gene panel. Genevestigator RefGenes [http://www.refgenes.org] is a publicly available online tool that uses a very large microarray database to identify genes that are most stable in conditions similar to that of your own experiment.

	== Reference genes across tissues ==		== Reference genes across tissues ==

Philip Zimmermann: /* Common but problematic references */

2010-07-05T07:44:28Z

Common but problematic references

←Older revision		Revision as of 07:44, 5 July 2010
Line 28:		Line 28:

	* Reference gene panels		* Reference gene panels
-	All genes used for normalization can show problems in one or the other condition. There are always conditions in which their expression differs significantly from their general level of expression. That is why reference genes should be validated for one's condition of interest. Ideally, one should choose from the complete genome rather than from a gene panel. Genevestigator RefGenes [http://www.genevestigator.com] is a publicly available online tool that uses a very large microarray database to identify genes that are most stable in conditions similar to that of your own experiment.	+	:All genes used for normalization can show problems in one or the other condition. There are always conditions in which their expression differs significantly from their general level of expression. That is why reference genes should be validated for one's condition of interest. Ideally, one should choose from the complete genome rather than from a gene panel. Genevestigator RefGenes [http://www.genevestigator.com] is a publicly available online tool that uses a very large microarray database to identify genes that are most stable in conditions similar to that of your own experiment.

	== Reference genes across tissues ==		== Reference genes across tissues ==

Philip Zimmermann: /* Common but problematic references */

2010-07-05T07:43:47Z

Common but problematic references

←Older revision		Revision as of 07:43, 5 July 2010
Line 26:		Line 26:
	* ribosomal RNAs ('''28S or 18S''')		* ribosomal RNAs ('''28S or 18S''')
	:'''[[User:Ajeffs\|Ajeffs]]''' 06:55, 21 April 2007 (EDT): 18S is generally a terrible choice for a reference gene thanks to the combination of (i) high abundance (creating a 1:100 dilution of template to run in parallel with neat template just for 18S is a complete drag); and (ii) having different degradation characteristics to mRNAs (it appears to be more resistant to degradation). However, if you can show that you have screened 5-10 reference genes, and 18S is still the best for your specific situation then so be it (but do try 28S if you or you PI is hung-up on 18S).		:'''[[User:Ajeffs\|Ajeffs]]''' 06:55, 21 April 2007 (EDT): 18S is generally a terrible choice for a reference gene thanks to the combination of (i) high abundance (creating a 1:100 dilution of template to run in parallel with neat template just for 18S is a complete drag); and (ii) having different degradation characteristics to mRNAs (it appears to be more resistant to degradation). However, if you can show that you have screened 5-10 reference genes, and 18S is still the best for your specific situation then so be it (but do try 28S if you or you PI is hung-up on 18S).
		+
		+	* Reference gene panels
		+	All genes used for normalization can show problems in one or the other condition. There are always conditions in which their expression differs significantly from their general level of expression. That is why reference genes should be validated for one's condition of interest. Ideally, one should choose from the complete genome rather than from a gene panel. Genevestigator RefGenes [http://www.genevestigator.com] is a publicly available online tool that uses a very large microarray database to identify genes that are most stable in conditions similar to that of your own experiment.

	== Reference genes across tissues ==		== Reference genes across tissues ==

Jakob Suckale: /* Reference genes across tissues */ Tub found in all tissues

2008-02-08T13:17:14Z

Reference genes across tissues: Tub found in all tissues

←Older revision		Revision as of 13:17, 8 February 2008
Line 34:		Line 34:
	* genes with the largest range (unsuitable for cross-tissue comparison): HPRT, Alb, PBGD, GAPDH, β2M		* genes with the largest range (unsuitable for cross-tissue comparison): HPRT, Alb, PBGD, GAPDH, β2M

-	* genes undetectable in tissue: Alb - colon; PPIA - ovaries; HPRT - prostate, testis, ovary, small intestine, colon; PBL, skeletal muscle; ~~Tub - ovaries,~~ PBGD - skeletal muscle; TBP - lung, prostate, colon; G6PDH - colon	+	* genes undetectable in tissue: Alb - colon; PPIA - ovaries; HPRT - prostate, testis, ovary, small intestine, colon; PBL, skeletal muscle; PBGD - skeletal muscle; TBP - lung, prostate, colon; G6PDH - colon
-	* genes detected in all tissues: GAPDH, Act, β2M, L13, PLA, RP2	+	* genes detected in all tissues: GAPDH, Act, Tub, β2M, L13, PLA, RP2
	(note the source Fig 2 is sometimes impossible to read and the describing text is incomplete; that might have lead to some errors above)		(note the source Fig 2 is sometimes impossible to read and the describing text is incomplete; that might have lead to some errors above)

	== Stability ==		== Stability ==
	*'''[[User:Ajeffs\|Ajeffs]]''' 06:55, 21 April 2007 (EDT): In addition to the given requirements of good (well, acceptable) specificity and efficiency of the reference gene primers, the next most important aspect of reference gene selection is stability. I don't care if the CT value of my reference genes (yes, genes, not gene) is close to the target genes/s or not - as long as the efficiency of all the primers is similar, and they are all working within their respective limits of detection i.e. linear range, then the stability of the reference genes between samples, treatments, etc. is the most crucial aspect of generating believable qPCR results.		*'''[[User:Ajeffs\|Ajeffs]]''' 06:55, 21 April 2007 (EDT): In addition to the given requirements of good (well, acceptable) specificity and efficiency of the reference gene primers, the next most important aspect of reference gene selection is stability. I don't care if the CT value of my reference genes (yes, genes, not gene) is close to the target genes/s or not - as long as the efficiency of all the primers is similar, and they are all working within their respective limits of detection i.e. linear range, then the stability of the reference genes between samples, treatments, etc. is the most crucial aspect of generating believable qPCR results.

Jakob Suckale: moved some content to main QRT-PCR page

2008-02-08T10:27:12Z

moved some content to main QRT-PCR page

←Older revision		Revision as of 10:27, 8 February 2008
Line 40:		Line 40:
	== Stability ==		== Stability ==
	*'''[[User:Ajeffs\|Ajeffs]]''' 06:55, 21 April 2007 (EDT): In addition to the given requirements of good (well, acceptable) specificity and efficiency of the reference gene primers, the next most important aspect of reference gene selection is stability. I don't care if the CT value of my reference genes (yes, genes, not gene) is close to the target genes/s or not - as long as the efficiency of all the primers is similar, and they are all working within their respective limits of detection i.e. linear range, then the stability of the reference genes between samples, treatments, etc. is the most crucial aspect of generating believable qPCR results.		*'''[[User:Ajeffs\|Ajeffs]]''' 06:55, 21 April 2007 (EDT): In addition to the given requirements of good (well, acceptable) specificity and efficiency of the reference gene primers, the next most important aspect of reference gene selection is stability. I don't care if the CT value of my reference genes (yes, genes, not gene) is close to the target genes/s or not - as long as the efficiency of all the primers is similar, and they are all working within their respective limits of detection i.e. linear range, then the stability of the reference genes between samples, treatments, etc. is the most crucial aspect of generating believable qPCR results.
-
-	~~== Selection ==~~
-	*'''[[User:Ajeffs\|Ajeffs]]''' 06:55, 21 April 2007 (EDT): Screen a handful of ref genes, select the most stable using genorm, bestkeeper etc, use at least 2 reference genes for subsequent reactions and normalisation. Inlcude your genorm M values when publishing qPCR data.

Jakob Suckale at 10:22, 8 February 2008

2008-02-08T10:22:35Z

←Older revision		Revision as of 10:22, 8 February 2008
Line 1:		Line 1:
-	'''Which reference genes should ~~be used~~ for a qPCR experiment?''' Quantifying mRNA via cDNA levels as in a [[QRT-PCR]] hinges on the references you choose. If you pick only one reference gene and your pick is not constant across different conditions or samples, your results will be skewed. Pick several and check whether they satisfy the criteria for a good reference gene.	+	'''Which reference genes should I use for my qPCR experiment?''' Quantifying mRNA via cDNA levels as in a quantitative reverse transcriptase PCR ('''[[QRT-PCR]]''') hinges on the references you choose. If you pick only one reference gene and your pick is not constant across different conditions or samples, your results will be skewed. Pick several and check whether they satisfy the criteria for a good reference gene.

-		+	== The ideal reference gene ==
-		+
-	~~== Normalisation with reference genes ==~~	+
-		+
-	=== The ideal reference gene ===	+
	A mRNA used as reference or standard of a [[QRT-PCR]] (and other experiments) should have the following properties:		A mRNA used as reference or standard of a [[QRT-PCR]] (and other experiments) should have the following properties:
	* expressed in all cells		* expressed in all cells
Line 11:		Line 7:
	* medium copy number for more accuracy (or similar copy number to gene of interest)		* medium copy number for more accuracy (or similar copy number to gene of interest)

-	=== Common reference genes ===	+	== Common reference genes ==
-	~~Common reference mRNAs linked to known mouse primer pairs:~~	+
	* ribosomal proteins (e.g. RPLP0)		* ribosomal proteins (e.g. RPLP0)
	* PPIA peptidylprolyl isomerase A = cyclophilin A		* PPIA peptidylprolyl isomerase A = cyclophilin A
Line 25:		Line 20:
	* HPRT1 hypoxanthine-guanine phosphoribosyltransferase		* HPRT1 hypoxanthine-guanine phosphoribosyltransferase

-	===Common but problematic references===	+	==Common but problematic references==
-	* glyceraldehyde-3-phosphate dehydrogenase GAPDH (common metabolic enzyme) [http://medgen.ugent.be/rtprimerdb/search_results.php?primer=&application=-1&detection=-1&locuslink=&snp=&last_name=&order=0&search=Search+Primers&first_result=0&pubmed=&string_type=substring&gene=gapdh&organism=2&Search=Search&id=], [http://www.realtimeprimers.org/SYBR%20Green/Mouse%20SYBR%20Green.html] - see [[QRT-PCR#Reference_mRNAs]]	+	* glyceraldehyde-3-phosphate dehydrogenase '''GAPDH''' (common metabolic enzyme) [http://medgen.ugent.be/rtprimerdb/search_results.php?primer=&application=-1&detection=-1&locuslink=&snp=&last_name=&order=0&search=Search+Primers&first_result=0&pubmed=&string_type=substring&gene=gapdh&organism=2&Search=Search&id=], [http://www.realtimeprimers.org/SYBR%20Green/Mouse%20SYBR%20Green.html] - see [[QRT-PCR#Reference_mRNAs]]
		+	:GAPDH has many functions besides the most well known in the glycolytic pathway [http://en.wikipedia.org/wiki/Glyceraldehyde_3-phosphate_dehydrogenase#Additional_functions]. Its levels are not constant [Zhu 2001 PMID 11237753] and vary more than for other genes across different tissues [Radonic 2004 PMID 14706621]. Problems using GAPDH as a qPCR reference gene have been published previously [Ke 2000 PMID 10799275, Suzuki 2000 PMID 10948434].

-	* ribosomal RNAs (28S or 18S)	+	* ribosomal RNAs ('''28S or 18S''')
-	:'''[[User:Ajeffs\|Ajeffs]] 06:55, 21 April 2007 (EDT):~~'''~~ 18S is generally a terrible choice for a reference gene thanks to the combination of (i) high abundance (creating a 1:100 dilution of template to run in parallel with neat template just for 18S is a complete drag); and (ii) having different degradation characteristics to mRNAs (it appears to be more resistant to degradation). However, if you can show that you have screened 5-10 reference genes, and 18S is still the best for your specific situation then so be it (but do try 28S if you or you PI is hung-up on 18S).	+	:'''[[User:Ajeffs\|Ajeffs]]''' 06:55, 21 April 2007 (EDT): 18S is generally a terrible choice for a reference gene thanks to the combination of (i) high abundance (creating a 1:100 dilution of template to run in parallel with neat template just for 18S is a complete drag); and (ii) having different degradation characteristics to mRNAs (it appears to be more resistant to degradation). However, if you can show that you have screened 5-10 reference genes, and 18S is still the best for your specific situation then so be it (but do try 28S if you or you PI is hung-up on 18S).

	== Reference genes across tissues ==		== Reference genes across tissues ==
Line 41:		Line 37:
	* genes detected in all tissues: GAPDH, Act, β2M, L13, PLA, RP2		* genes detected in all tissues: GAPDH, Act, β2M, L13, PLA, RP2
	(note the source Fig 2 is sometimes impossible to read and the describing text is incomplete; that might have lead to some errors above)		(note the source Fig 2 is sometimes impossible to read and the describing text is incomplete; that might have lead to some errors above)
-
-	~~== Primer collections ==~~
-
-	~~Search primer repositories like [http://medgen.ugent.be/rtprimerdb/ RTPrimerDB] (see also below) and check the literature before doing it from scratch. <br>~~
-	~~Check out the Eccles Lab collection of human and mouse [[Eccles:QPCR_reference_genes\| qPCR reference genes]] on OWW.~~

	== Stability ==		== Stability ==
-	*'''[[User:Ajeffs\|Ajeffs]] 06:55, 21 April 2007 (EDT):~~'''~~ In addition to the given requirements of good (well, acceptable) specificity and efficiency of the reference gene primers, the next most important aspect of reference gene selection is stability. I don't care if the CT value of my reference genes (yes, genes, not gene) is close to the target genes/s or not - as long as the efficiency of all the primers is similar, and they are all working within their respective limits of detection i.e. linear range, then the stability of the reference genes between samples, treatments, etc. is the most crucial aspect of generating believable qPCR results.	+	*'''[[User:Ajeffs\|Ajeffs]]''' 06:55, 21 April 2007 (EDT): In addition to the given requirements of good (well, acceptable) specificity and efficiency of the reference gene primers, the next most important aspect of reference gene selection is stability. I don't care if the CT value of my reference genes (yes, genes, not gene) is close to the target genes/s or not - as long as the efficiency of all the primers is similar, and they are all working within their respective limits of detection i.e. linear range, then the stability of the reference genes between samples, treatments, etc. is the most crucial aspect of generating believable qPCR results.

	== Selection ==		== Selection ==
-	*'''[[User:Ajeffs\|Ajeffs]] 06:55, 21 April 2007 (EDT):~~'''~~ Screen a handful of ref genes, select the most stable using genorm, bestkeeper etc, use at least 2 reference genes for subsequent reactions and normalisation. Inlcude your genorm M values when publishing qPCR data.	+	*'''[[User:Ajeffs\|Ajeffs]]''' 06:55, 21 April 2007 (EDT): Screen a handful of ref genes, select the most stable using genorm, bestkeeper etc, use at least 2 reference genes for subsequent reactions and normalisation. Inlcude your genorm M values when publishing qPCR data.

Jakob Suckale: QRT-PCR normalisation moved to Choosing reference genes for qPCR normalisation

2008-02-08T10:05:56Z

QRT-PCR normalisation moved to Choosing reference genes for qPCR normalisation

←Older revision

Revision as of 10:05, 8 February 2008

Jakob Suckale at 10:04, 8 February 2008

2008-02-08T10:04:04Z

←Older revision		Revision as of 10:04, 8 February 2008
Line 1:		Line 1:
-	Quantifying mRNA via cDNA levels as in a [[QRT-PCR]] hinges on the references you choose. If you pick only one reference gene and your pick is not constant across different conditions or samples, your results will be skewed. Pick several and check whether they satisfy the criteria for a good reference gene.	+	'''Which reference genes should be used for a qPCR experiment?''' Quantifying mRNA via cDNA levels as in a [[QRT-PCR]] hinges on the references you choose. If you pick only one reference gene and your pick is not constant across different conditions or samples, your results will be skewed. Pick several and check whether they satisfy the criteria for a good reference gene.

-	~~== Normalisation methods ==~~
-	There is an ongoing debate what is the best way to normalise qPCR data. Reference genes are the most common method, although single unverified reference genes invalidate the qPCR data generated. Total RNA, ribosomal RNA, and genomic DNA have been suggested as alternative methods.

-	~~=== Reference genes ===~~
-	Most common method. Best practise is a panel, e.g. [http://www.bioline.com/h_prod_detail.asp?user_prodname=Human%20Endogenous%20Control%20Gene%20Panel] not just a single reference gene and including data on suitability as reference genes. Often ''housekeeping gene''  is used here instead of reference gene but the term is poorly defined and can be misleading.

-	==~~= RNA ===~~	+	== Normalisation with reference genes ==
-	Total rRNA [http://scholar.google.com/scholar?hl=en&lr=&safe=off&cluster=12435126891737656303] [http://scholar.google.com/scholar?hl=en&lr=&safe=off&cluster=9547016096229453970], or total RNA. Drawback: rapidly dividing cells will have more rRNA and different rRNA/mRNA ratio which will complicate comparison; difference in cDNA synthesis not taken into account.	+
-		+
-	~~=== Genomic DNA ===~~	+
-	~~Genomic DNA or cell number. Drawbacks: RNA degrades faster than RNA which can distort the data; sample cannot be DNase treated; efficiency of cDNA synthesis not taken into account.~~	+
-		+
-	~~== The ideal~~ reference ~~gene~~ ==	+

		+	=== The ideal reference gene ===
	A mRNA used as reference or standard of a [[QRT-PCR]] (and other experiments) should have the following properties:		A mRNA used as reference or standard of a [[QRT-PCR]] (and other experiments) should have the following properties:
	* expressed in all cells		* expressed in all cells
Line 20:		Line 11:
	* medium copy number for more accuracy (or similar copy number to gene of interest)		* medium copy number for more accuracy (or similar copy number to gene of interest)

-	== Common reference genes ==	+	=== Common reference genes ===
-		+
	Common reference mRNAs linked to known mouse primer pairs:		Common reference mRNAs linked to known mouse primer pairs:
	* ribosomal proteins (e.g. RPLP0)		* ribosomal proteins (e.g. RPLP0)
Line 35:		Line 25:
	* HPRT1 hypoxanthine-guanine phosphoribosyltransferase		* HPRT1 hypoxanthine-guanine phosphoribosyltransferase

-	Common but ~~not recommended~~ references	+	===Common but problematic references===
	* glyceraldehyde-3-phosphate dehydrogenase GAPDH (common metabolic enzyme) [http://medgen.ugent.be/rtprimerdb/search_results.php?primer=&application=-1&detection=-1&locuslink=&snp=&last_name=&order=0&search=Search+Primers&first_result=0&pubmed=&string_type=substring&gene=gapdh&organism=2&Search=Search&id=], [http://www.realtimeprimers.org/SYBR%20Green/Mouse%20SYBR%20Green.html] - see [[QRT-PCR#Reference_mRNAs]]		* glyceraldehyde-3-phosphate dehydrogenase GAPDH (common metabolic enzyme) [http://medgen.ugent.be/rtprimerdb/search_results.php?primer=&application=-1&detection=-1&locuslink=&snp=&last_name=&order=0&search=Search+Primers&first_result=0&pubmed=&string_type=substring&gene=gapdh&organism=2&Search=Search&id=], [http://www.realtimeprimers.org/SYBR%20Green/Mouse%20SYBR%20Green.html] - see [[QRT-PCR#Reference_mRNAs]]
-	* ribosomal RNAs (28S or 18S) - ~~see below~~	+
		+	* ribosomal RNAs (28S or 18S)
		+	:'''[[User:Ajeffs\|Ajeffs]] 06:55, 21 April 2007 (EDT):''' 18S is generally a terrible choice for a reference gene thanks to the combination of (i) high abundance (creating a 1:100 dilution of template to run in parallel with neat template just for 18S is a complete drag); and (ii) having different degradation characteristics to mRNAs (it appears to be more resistant to degradation). However, if you can show that you have screened 5-10 reference genes, and 18S is still the best for your specific situation then so be it (but do try 28S if you or you PI is hung-up on 18S).

	== Reference genes across tissues ==		== Reference genes across tissues ==
Line 59:		Line 51:

	== Selection ==		== Selection ==
-	*'''[[User:Ajeffs\|Ajeffs]] 06:55, 21 April 2007 (EDT):''' Screen a handful of ref genes, select the most stable using genorm, bestkeeper etc, use at least 2 reference genes for subsequent reactions and normalisation. Inlcude your genorm M values when publishing qPCR data.	+	*'''[[User:Ajeffs\|Ajeffs]] 06:55, 21 April 2007 (EDT):''' Screen a handful of ref genes, select the most stable using genorm, bestkeeper etc, use at least 2 reference genes for subsequent reactions and normalisation. Inlcude your genorm M values when publishing qPCR data.
-		+
-	~~== Use of 18S ==~~	+
-	*'''[[User:Ajeffs\|Ajeffs]] 06:55, 21 April 2007 (EDT):''' 18S is generally a terrible choice for a reference gene thanks to the combination of (i) high abundance (creating a 1:100 dilution of template to run in parallel with neat template just for 18S is a complete drag); and (ii) having different degradation characteristics to mRNAs (it appears to be more resistant to degradation). However, if you can show that you have screened 5-10 reference genes, and 18S is still the best for your specific situation then so be it (but do try 28S if you or you PI is hung-up on 18S).	+