Chemical structures of some small-molecule effectors of the lac operon. Like allolactose, IPTG (isopropyl β-d-1-thiogalactopyranoside) can bind the Lac repressor and cause its dissociation from the operator, inducing transcription of the lac operon. However, IPTG is not a substrate for β-galactosidase. The β-galactoside X-gal (5-bromo-4-chloro-3-indolyl-β-d-galactopyranoside) does not induce expression of the lac operon, but it does serve as an experimentally useful substrate for β-galactosidase, producing a blue color when metabolized.