Control of body segment differentiation in Drosophila by the bithorax gene complex.

The bithorax complex is a gene cluster whose function is to regulate still other genes in such a way that each segment of the body commencing with the second thoracic segment and proceeding posteriorly, develops a unique pattern of structures. Numerous cis-regulatory regions have been identified within the complex and are believed to bind to repressor-like substances elaborated by the wild-type alleles of at least two major trans-regulatory genes, Polycomb and extra sex combs. The more proximal is a gene in the complex (with the exception of postbithorax) the more likely it is to be derepressed; the more posterior is a segment in the organism, the greater is the number of BX-C genes that become derepressed in that segment. A gradient in repressor substance(s) and a gradient in affinity of the cis-regulatory regions for repressor can account in a formal sense for the regulation of the BX-C genes themselves.