SRD5A2 and benign prostatic hyperplasia: 5α-Reductase inhibitors (5ARIs) including finasteride and dutasteride (Fig. 1b), which mainly target SRD5A2, but also act on other SRD5As13, have been used as a major class of antiandrogenic drugs to treat BPH and androgenic alopecia1,7,12,14, and are indicated in the treatment of prostate cancer15.