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In situ hybridization studies reveal novel sites of expression of cholesterol side-chain cleavage cytochrome P450 (P450scc) during murine embryonic development. In addition to fetal adrenals and testes, P450scc transcripts localize in situ to the primitive gut and to a subset of unidentified cells in the dermal mesenchyme of embryonic skin. In the gut, transcripts are most abundant in luminal epithelia of the hindgut, which will form the colon. P450scc transcript abundance at these novel sites is a fraction of that in fetal adrenals or testes, suggesting a local rather than an endocrine function. Immunocytochemical analyses localize P450scc protein to the fetal hindgut, indicating that the transcripts are translated in vivo. RNA isolated from microdissected embryonic hindgut and skin was reverse transcribed and amplified by polymerase chain reaction. DNA sequence analyses of polymerase chain reaction products confirmed that specific hybridization in situ represents authentic P450scc gene (Cyp11A) transcripts and that 3 beta-hydroxysteroid dehydrogenase/delta 5-->delta 4-isomerase transcripts are also present, demonstrating the potential of these fetal tissues to produce pregnenolone and progesterone. P450scc transcripts are also detectable by in situ hybridization in primitive gut and skin of Fushi tarazu factor 1 null mice, which lack the nuclear receptor steroidogenic factor 1, proving that steroidogenic factor 1 is not required for steroid hydroxylase gene expression at these sites. The capacity for C21 steroid biosynthesis in primitive gut and skin during organogenesis raises the question whether local production of steroid hormones may be required for normal cellular growth and differentiation of these tissues during embryogenesis.