The murine homologue of the 25-hydroxyvitamin D [25(OH)D] 1α-hydroxylase gene [1α(OH)ase; Cyp27b1], which is mutated in humans with vitamin D-dependent rickets type I (VDDR-I; also known as pseudovitamin D-deficiency rickets [PDDR]) was cloned and characterized. Like the human, the mouse gene has nine exons, and the exon-intron organization is well conserved. By interspecific backcross analysis, the Cyp27b1 gene was mapped to 70.5 cM on mouse Chr 10. This is in a region syntenic with human Chr 12q13.1-q13.3 to which the human 1α(OH)ase gene was previously mapped. Kidney expression of the 1α(OH)ase was localized to cortical tubules and was higher in the adult mouse than in the fetus, consistent with the increased role of its product as a circulating hormone postnatally. Prenatally, the 1α(OH)ase gene, together with the vitamin D receptor (VDR) gene, was expressed in embryonic stem cells, and expression of 1α(OH)ase in bone and intestine was higher in the fetus than in the adult. These observations suggest that 1,25-dihydroxyvitamin D [1,25(OH)2D] plays a role in fetal development. In view of the fact that humans lacking 1α(OH)ase have apparently normal prenatal development, this may point to functional redundancy in the fetal vitamin D system, which now can be explored further in mouse models in which the 1α(OH)ase gene has been deleted.