X-gal stock solution (25 mg/ml)

X-gal (5-bromo-4-chloro-3-indolyl-b-D-galactopyranoside; e.g., Invitrogen 15520034)

Dimethylformamide (DMF; e.g., Sigma D8654)

Prepare a solution of 25 mg/ml X-gal in DMF. X-gal is very expensive and should be used carefully. Use it only with glass containers and glass pipettes. Store at -20°C protected from light.