In hypogonadal men, testosterone treatment increases prostate size and PSA.[15, 16] Exogenous testosterone has been shown to stimulate growth of metastatic prostate cancer,[24] and androgen-deprivation is used to treat metastatic and locally advanced prostate cancer.[18] However, it is unclear if testosterone treatment increases the risk for incident prostate cancer or if it alters the natural history of pre-existing organ-confined prostate cancer. Here, KLK3 is linked to prostate cancer.