The document discusses the lac operon, which was first described by Francois Jacob and Jacques Monod in 1961. The lac operon includes the lacI gene, which encodes the lac repressor protein. The lac repressor protein regulates the lac operon by binding to the operator region and preventing transcription of the structural genes under normal conditions. When lactose is present, it binds to the lac repressor and causes a conformational change that prevents it from binding to the operator, allowing transcription to occur. The lac repressor is a tetramer protein made up of four identical 360 amino acid subunits with a total molecular weight of around 154,520 Daltons.