IPTG and Xgal for blue/white selection

Stock Solutions

IPTG : isopropyl thiogalactoside, or isopropyl beta-D-thiogalactopyranoside. Sigma stock number I5502.
     I use a 0.1 M solution. The formula weight is 238.3, so this is 0.238 g in 10 ml of water. Sterilize by filtration, then store in the freezer.
  
Xgal : 5-bromo-4-chloro-3-indolyl-beta-D-galactopyranoside. Sigma stock number B4252.
     I use a 20 mg/ml solution. It must be dissolved in DMSO (dimethyl sulfoxide) or dimethyl formamide, not water! It must be wrapped in foil to protect it from the light, and stored in the freezer. According to Maniatis et al., Xgal solutions do not need to be sterilized.

 

Using IPTG and Xgal for blue/white selection on Petri plates

There are three basic methods: spread the chemicals on top of the plates before you use them, pour the plates with IPTG and Xgal in them, or incorporate the chemicals into top agar.