The function of β-galactosidase in lactose metabolism is to hydrolyze lactose to form glucose and galactose. If the growth medium contains both glucose and lactose, then in the interest of efficiency there is no need for a cell to turn on the lac operon. Experiments performed by Monod in the mid-1940s demonstrated that cells behave according to this logic. E. coli cells incubated in the presence of glucose and lactose do not start to make β-galactosidase until all of the exogenous glucose is consumed. These findings were later extended to lactose permease and transacetylase, the two other proteins specified by the lac operon. As also the reason that lactose enzymes are not made when glucose is present is that no lac NRNA is made. Transcription-level inhibition of the lactose enzymes and a variety of other inducible enzymes by glucose (or other readily used carbon sources) is called catabolite repression.