LAC repressor binding to DNA. Computer graphic of a lac repressor molecule (pink) interacting with genes on DNA that control lactose metabolism in Escherichia coli bacteria. The lac (lactose) repressor molecule is a protein made of four units and two arms. Lac arms are seen binding to two of three gene sites on a spiral DNA (Deoxyribonucleic acid) molecule. The bound DNA forms a repression loop, trapping and silencing a blue CAP activator protein. Switching off these genes occurs when lactose levels in E.coli are low. The genes code for an enzyme which metabolizes lactose. This gene regulation may help the design of gene therapies.