CYP19A1 and benign prostatic hyperplasia: Aromatase, the enzyme required for metabolism of androgens to estrogens, is expressed in the stroma of the normal prostate.[18] The absence of aromatase activity in both the stroma and epithelium of prostate tissue recombinants results in epithelial hyperplasia comparable to that seen in intact ArKO mice and in men with benign prostatic hyperplasia.[19] The absence of local aromatase expression in the prostate is a key factor in determining epithelial cell hyperplasia.