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Cytochrome P450 aromatase (P450arom) is an enzyme responsible for the conversion of androgen into oestrogen. To study the role of ovarian aromatase (Cyp19a1a) in sex differentiation, the novel c DNA clone encoding ovarian aromatase (Cyp19a1a) was identified from the ovary of the black rockfish ( Sebastes schlegeli). The cloned c DNA is 1868 bp in length with 79 bp 5′U TR, 209 bp 3′UTR [excluding poly (A)] and contains 1550 bp ORF, encoding a protein of 516 amino acids. Alignment and phylogenetic analysis showed that the black rockfish ovarian aromatase (Cyp19a1a) shared 61-89% sequence identity with those of other seven fish species. The cyp19a1a transcript was found more highly abundant in ovary, but not detected in the testis, brain, liver, spleen, kidney, muscle, skin, heart, head kidney, gill, intestine and eyes by semi-quantitative reverse transcription ( RT)- PCR. The cyp19a1a gene is not equivalent in tissue-specific expression in the female and male, showed the possibility of distinct promoters and regulatory mechanisms. The expression level of cyp19a1a in ovary declined with the ovary development, but was higher than that in testis in all phases. These results indicated that cyp19a1a might be involved in gonadal development in S. schlegeli. [ABSTRACT FROM AUTHOR]