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Cytochrome P450c17 encoded by CYP17, whose expression is regulated by peptide hormones via cAMP, is required for cortisol and sex hormone biosynthesis thereby playing a key role in biological processes including sexual differentiation. Utilizing the cAMP-regulatory sequence CRS1 of the bovine CYP17 gene as an affinity ligand, four CRS1-binding proteins have been purified from nuclear extracts of mouse adrenocortical Y1 cells and shown to enhance the in vitro transcription of a reporter gene promoted by CRS1. Microsequencing of these four proteins established two of them to be the homeodomain proteins Pbx1a and Pbx1b, originally discovered by their involvement in the t(1;19) chromosomal translocation in pre-B-cell acute lymphoblastic leukemias. Overexpression of Pbx1 in Y1 cells enhances cAMP-dependent transcription of the CRS1-dependent reporter gene. These results identify the CRS1 of bovine CYP17 as a cellular target for Pbx1 and suggest that one role of this homeodomain protein is in the regulation of steroidogenesis and subsequently sexual development.