The enzyme steroid 5α-reductase catalyzes the conversion of testosterone into the more powerful androgen, dihydrotestosterone. We previously described the cloning of rat and human cDNAs that encode steroid 5α-reductase and their expression in oocytes and cultured cells. Here, we report the isolation, characterization, and chromosomal mapping of two human steroid 5α-reductase genes. One gene (symbol SRD5A1) is functional, contains five exons separated by four introns, and maps to the distal short arm of chromosome 5. Two informative restriction fragment length polymorphisms are present in exons 1 and 2 of this gene. A second gene (symbol SRD5AP1) has all of the hall-marks of a processed pseudogene and was mapped to the q24-qter region of the X chromosome. In the mouse, a single steroid 5α-reductase gene (Srd5α-1) is linked to Xmv-13 on chromosome 13.
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