Isopropyl β-D-1-thiogalactopyranoside (IPTG) is widely used in molecular biology as a molecular mimic of allolactose, which induces the expression of genes under the control of the lac operon. It is particularly useful in the induction of protein expression in bacterial systems, such as E. coli, where it binds to the lac repressor, causing it to release from the operator region of the DNA. This allows RNA polymerase to transcribe the downstream genes, including those encoding for recombinant proteins. IPTG is favored over lactose for this purpose because it is not metabolized by the cells, ensuring a consistent level of induction. Additionally, it is used in blue-white screening to identify recombinant bacteria that have successfully incorporated a plasmid with an insert, as IPTG induces the production of β-galactosidase, which can cleave X-gal to produce a blue product in the absence of an insert. This makes IPTG an essential tool in cloning, protein production, and genetic engineering research.