François Jacob and Jacques Monod, at the Pasteur Institute in Paris, together with André Lwoff, formulate in 1960 the operon model to explain how bacteria turn specific genes on or off in response to changes in their environment — for example, producing the enzymes needed to digest a particular sugar only when that sugar is present in the medium, rather than constantly manufacturing every possible enzyme. The model proposes that functionally related genes are grouped into units called operons, controlled by a regulatory region where a repressor protein binds; when a signal molecule — such as the presence of the sugar lactose — binds to that repressor protein, it is released from the DNA and allows the operon's genes to be transcribed. The key experiment, known as the PaJaMo experiment (from the initials of Pardee, Jacob, and Monod), experimentally demonstrates this genetic control mechanism through the transfer of genetic material between bacterial strains. The operon model establishes the general principle that gene activity is not fixed but regulable, laying the conceptual foundations for all subsequent molecular biology of gene regulation, including the understanding of how cells in more complex organisms — including humans — turn specific genes on and off during development and in response to stimuli.