Within the serial array of vertebrate homeobox genes in the Hox complexes, it is possible to define a subgroup that is structurally homologous to the Drosophila homeotic gene Deformed (Dfd). We wished to test whether a vertebrate Dfd-like protein could substitute for any of the regulatory functions of the Dfd protein in Drosophila embryos, including its ability to transcriptionally activate the Dfd transcription unit. A fusion gene consisting of a heat shock promoter attached to the human Hox-4.2 gene was introduced into the Drosophila genome, and its regulatory and developmental effects were assayed after heat shock. In developing embryonic and larval cells, we find that human Hox-4.2 specifically activates ectopic expression of the endogeneous Dfd transcription unit and phenocopies a dominant mutant allele of Dfd. Thus, human Hox-4.2 can specifically substitute for a normal regulatory function of its Drosophila homolog, Dfd.