The Cluster randomised triAl of PSA testing for Prostate cancer (CAP) trial is a population-based cluster randomised controlled trial in England and Wales assessing the impact of a single invitation for PSA testing for prostate cancer on prostate cancer mortality.9 Within this trial, trained researchers extract detailed data from the medical records of men who have died with or of prostate cancer onto a structured pro forma. This evidence concerns the gene KLK3 and prostate carcinoma.