1940-

American molecular biologist who was the first to identify and describe the operation of repressor genes in an organism. This discovery, made simultaneously and independently of Walter Gilbert's work, helped to shed additional light on the workings of the genome. Ptashne demonstrated, for example, that an enzyme responsible for digesting lactose is turned off by a repressor gene when lactose is absent from the cell. This process is reversible when lactose exists in the cell.