pET is an expression vector engineered to produce copies of a selected protein in a host cell.

A viral T7 RNA polymerase gene is controlled by a lac repressor (lacI) and operator (lacO), making it inducible by the lactose analog IPTG.

Growing host cells on IPTG derepresses the T7 RNA polymerase gene.

The RNA polymerase binds to its promoter and transcribes the target gene.