Reduced aromatase activity may lead to ovarian hyperandrogenism and the development of PCOS, which can be deduced from the facts that a higher frequency of PCOS is observed in people with aromatase deficiency caused by rare loss-of-function mutations [16-18] and antral follicles taken from PCOS women exhibits no aromatase activity [19]. The gene discussed is CYP19A1; the disease is hyperinsulinemic hypoglycemia, familial, 4.