THIS article is an account of how basic research on the nature of the gene has led to the
discovery of two clusters of master control genes that play a central role in programming the
development of the fly. These clusters, known as the bithorax complex (BX-C) and Antenna-
pedia complex (ANT-C), are believed to have evolved from an ancestral gene by a process
of tandem duplication and di-vergence in function by mutation. So successful during
evolution have cognates of these clusters been in program-ming development that they are …