X-Gal is the common short name for 5-bromo-4-chloro-3-indolyl-β-D-galactopyranoside. It is a substrate for beta-galactosidase (lacZ) and turns blue upon being cleaved. It is commonly used for blue-white screening.
A stock solution of X-gal is commonly made with 20 mg/ml X-gal in DMF (dimethylformamide). When pouring agar plates, a typical final concentration is 20 μg/ml.