IPTG (isopropyl beta-D-1-thiogalactopyranoside) induces the transcription of genes from the lac and tac operons in bacteria, notably the hydrolase enzyme beta-galactosidase (ß-Gal). Once expressed, ß-Gal hydrolyzes beta-galactoside and lactose sugars into monosaccharides. IPTG is commonly used in the beta-galactosidase assay, wherein cells transfected with vector carrying the E. coli-derived lacZ gene, which encodes ß-Gal, are lysed and analyzed via the reaction substrate O-nitrophenyl-beta-D-galactopyranoside (ONPG). The ONPG substrate used in the beta-galactosidase assay turns yellow upon cleavage by ß-Gal, allowing quantitation of the transfected ß-Gal in cells and its use as a transfection control.