5α-reductase inhibitors (5ARIs) including finasteride and dutasteride (Fig. 1b), which mainly target SRD5α2 but also act on other SRD5αs13, have been used as a major anti-androgenic class of drugs to treat BPH and androgenic alopecia1,7,12,14 and indicated in the treatment of prostate cancer15. Here, SRD5A2 is linked to benign prostatic hyperplasia.