Domains of cis-Interaction in the Bithorax Complex

ogaster specifies the proper determination of segments in the fly. The third thoracic segment and the first through the eighth abdominal segments are affected by various mutations within the complex. Recessive loss-of-function mutations transform a segment or seg-ments to resemble a more anterior segment; dominant gain-of-function alleles transform a segment to resem-ble a more posterior one. The BX-C is the largest clus-ter of homeotic mutations in Drosophila, and thus it is a particularly appealing locus for a molecular study of developmental decisions. DNA of the BX-C was first isolated 6 years ago (Bender et al. 1983a), but we still have only preliminary information on one group of gene products (White and Wilcox 1984; Beachy et al. 1985). We have no idea how