X-gal solution (2% w/v)
X-gal is 5-bromo-4-chloro-3-indolyl-β-D-galactosidase.
Make a stock solution by dissolving x-gal dimethyl formamide at a concentration of 20mg/ml solution. Use a glass or polypropylene tube. Wrap the tube containing the solution in aluminium foil to prevent damage by light and store at -200C.
It is not necessary to sterilize x-gal solution by filtration.