The transgene contains the ovary- and testis-specific promoter of the human cytochrome P450, family 19, subfamily a, polypeptide 1 (CYP19A1) gene, comprising 278 base pairs (bp) of 5' flanking DNA and 26 bp of the alternatively spliced exon IIa, driving expression of mammalian codon-improved Cre recombinase (icre); the coding sequence is followed by the SV40 polyadenylation signal. RT-PCR analysis of various tissues (heart, muscle, lung, spleen, brain, liver, pancreas, ovary, testis) demonstrates Cre recombinase-mediated excision of a loxP-flanked sequence only in the ovaries and testes of transgenic mice. Histologic analysis of X-gal-stained testes of mice carrying this transgene and a lacZ Cre reporter allele (Gt(ROSA)26Sortm1Sor) reveal Cre recombinase activity from postpartum day 3 (P3) in Sertoli cells at the periphery of the seminiferous tubules and in the Leydig cells of the interstitium, but not in germ cells.