 Another important development concerned gene regulation. Studying the ability of bacteria to adapt to varying food sources, François Jacob (1920–2013) and Jacques Monod (1910–1976) developed the operon model, according to which a protein called “repressor” is permanently produced, which binds to a DNA region called “operator” in the vicinity of the genes required for lactose metabolism, thereby inhibiting the production of the respective enzymes and other proteins. If lactose was present, it (or a derivate) interacts with the repressor such that the genes are activated, and lactose could be digested by the bacteria. This model introduced an important distinction regarding genes. Those genes that coded for β-galactosidase or lactose permease or other proteins constituting cellular functions were called “structural genes”. The genes that produced the repressor and other proteins controlling the expression of genes were called “regulatory genes” (Jacob &amp; Monod 1961).