Immunohistochemical demonstration of steroid C21 hydroxylase in normal and neoplastic human breasts

Steroid C21 hydroxylase was investigated immunohistochemically with the use of antibody against cytochrome P‐450 specific for steroid C21 hydroxylation (P‐450_C21) in normal and neoplastic human breast tissues. In the histologically normal breast, P‐450_C21 was exclusively present in secretory tubules and ducts. In mammary dysplasia and fibroadenoma, P‐450_C21 was intensively stained in epithelial cells. In gynecomastia, P‐450_C21 was faintly observed in epithelial cells in some cases. In intraductal and invasive ductal carcinoma, P‐450_C21 was observed in the cells with ductal formation. P‐450_C21 was not observed in medullary and mucinous carcinoma. In lobular carcinoma, only two cases were positive for P‐450_C21 of nine cases examined. P‐450_C21 is considered to be closely related to the ductal differentiation in neoplastic transformation of the breast.