Gene regulation is essential for viruses, prokaryotes and eukaryotes as it increases the versatility and adaptability of an organism by allowing the cell to express protein when needed. Although as early as 1951, Barbara McClintock showed interaction between two genetic loci, Activator ("Ac") and Dissociator ("Ds"), in the color formation of maize seeds, the first discovery of a gene regulation system is widely considered to be the identification in 1961 of the "lac" operon, discovered by François Jacob and Jacques Monod, in which some enzymes involved in lactose metabolism are expressed by "E. coli" only in the presence of lactose and absence of glucose.