LAC repressor bound to DNA. Molecular model of a LAC (lactose) repressor molecule (pink and turquoise) interacting with bacterial DNA (deoxyribonucleic acid, red and blue). The LAC repressor inhibits the expression of genes that code for an enzyme which metabolizes lactose in bacteria. It is only present when lactose levels are low. This ensures that the bacteria only produce machinery used for the uptake and breakdown of lactose, when lactose is actually present.