We found that harmol hydrochloride, previously identified as a non-competitive inhibitor of AR [25,26] or a weak inhibitor of androgen signaling [24], is actually a competitive antagonist of AR that inhibits the growth of the VCaP prostate cancer line, at concentrations for which it did not affect the growth of AR negative DU145 and PC3 cells. The gene discussed is AR; the disease is prostate cancer.