Although this could be due to ascertainment bias (particularly given the higher prevalence of PSA testing in the UKB cohort compared with the general UK population [27, 28]), the difference in mean number of PSA tests was less than 1 between those with no diseases and those with ≥ 4 diseases, so PSA testing is unlikely to fully explain the apparent reduction in prostate cancer risk. This evidence concerns the gene KLK3 and Familial prostate cancer.