IPTG (Isopropyl ß-D-thiogalactopyranoside) is a lactose analog that mimics allolactose, which triggers transcription of the lac operon by inactivation of the lac repressor. As such, IPTG can be used to induce protein expression of heterologous proteins that are encoded by genes under the control of the lac operator.
X-gal (5-Bromo-4-chloro-3-indolyl-ß-D-galactopyranoside) is a synthetic analog of lactose that mimics lactose as an inert chromogenic substrate for ß-galactosidase.