Division of expression after gene duplication. Squares represent genes, closed ovals represent cis-acting elements that regulate gene transcription, and open ovals represent deactivated cis-elements. Consider a gene that is expressed in tissues T1 and T2, with a cis-acting regulatory element A1 controlling the expression in T1 and A2 controlling the expression in T2. Following gene duplication, one daughter gene might lose the A1 element whereas the other gene might lose A2, so that each is expressed in only one of the two tissues. (Adapted from Zhang [139].)