Genomic DNA encoding Sex-lethal, a developmental switch gene in Drosophila melanogaster that regulates sex determination and dosage compensation has been isolated. Wild-type DNA sequence organization of the gene has been compared at the restriction level with those of 17 female-specific, loss-of-function and five male-specific, gain-of-function mutant alleles. DNA lesions associated with 12 of these mutations delimit an 11 kb DNA region that is necessary for proper Sex-lethal function in females. Males who are deleted for this region are both viable and fertile. Loss-of-function alleles are associated with gross DNA alterations as well as true point mutations; the former are located throughout the region. In contrast, all five gain-of-function alleles are associated with DNA insertions that are clustered within a 1 kb portion of the Sxl gene region.