IPTG (Isopropyl β-D-1-thiogalactopyranoside) is a molecular compound commonly used in molecular biology, especially for protein expression in Escherichia coli (E. coli). It is a non-metabolizable lactose analog that induces the lac operon, activating gene expression under the control of T7 RNA polymerase or lac promoters. Unlike lactose, IPTG is not degraded by β-galactosidase, allowing sustained induction of gene expression. It is widely used in recombinant protein expression systems, such as the pET system.