Isopropyl-beta-D-Thiogalactopyranoside is an inducer of beta-galactosidase used to promote the expression of proteins in cells controlled by the lac operator system. When using blue/white screening, 0.1 mM final IPTG concentration in LB (Luria Broth) media is recommended. Isopropyl-?-D-Thiogalactopyranoside ( IPTG ) is usually used with X gal.