Each segment of Drosophila has a characteristic pattern of muscles. Like the segments of the cuticle and the central nervous system, the muscle pattern is ultimately dependent on the deployment of selector genes such as elements of the bithorax complex. We use nuclear transplantation to make genetic mosaics in which the donor, but not the host, is mutant for part of the bithorax complex. Making use of a muscle pattern that is found only in the male, we ask which cells have to be mutant in order to obtain mutant muscles and find that these crucial cells do not contribute to the muscles themselves. The evidence implicates neurons that innervate the muscles. Our hypothesis is that the sex and segmental identity of the motor or neurosecretory neurons determine the development of muscle pattern.