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Protocol Prepare a 20 mg/ml stock solution of 5-bromo-4-chloro-3-indolyl β-D-galactopyranoside (X-GAL) in N,Ndimethylformamide (DMF) or dimethylsulfoxide (DMSO). Procedures Blue/White Colony Screening a) X-Gal included in agar: Add 5 ml of X-Gal stock solution and 5 ml of 0.1 M isopropyl-b-D-thiogalactoside (IPTG, BIO-37082) for each 1L of autoclaved media agar (e.g. LB agar) containing appropriate antibiotics just before pouring. The media should be below 55 °C. Plate cells on cooled agar and incubate overnight at 37 °C. b) X-Gal applied to top of agar: To a premade LB agar plate (e.g. prepared using LB agar), add 40 µl of X-Gal stock solution (at room temperature) and 4 µl of a 200 mg/ml solution of IPTG. Spread solution over the entire surface of the plate. Incubate at 37 °C until the fluid is no longer visible. This may take several hours. Plate cells and incubate overnight at 37 °C.