The largest trial to date, the Cluster Randomised Trial of PSA Testing for Prostate Cancer (CAP), found no statistically significant difference in prostate cancer-specific mortality between controls and those in the study arm who were offered low-intensity screening with a one-off PSA test.9 However, the degree of contamination in the control arm was not empirically assessed; this is particularly important, given that the intervention arm’s screening uptake was only 36%. Here, KLK3 is linked to prostate cancer.