Effects of IPTG on lac repressor structure. The structure of the lac repressor bound to the inducer isopropylthiogalactoside (IPTG), shown in orange, is superimposed on the structure of the lac repressor bound to DNA, shown in purple. Notice that the binding of IPTG induces structural changes that alter the relation between the two DNA-binding domains so that they cannot interact effectively with DNA. The DNA-binding domains of the lac repressor bound to IPTG are not shown, because these regions are not well ordered in the crystals studied.