A textbook example of over-diagnosis and overtreatment as a by-product of testing with a relatively weak instrument is the use of prostate specific antigen (PSA) testing when diagnosing prostate cancer [10,11]: the introduction of PSA measurements for diagnosing prostate cancer led to a reduction of the death rate, but also to overdiagnosis and overtreatment. This evidence concerns the gene KLK3 and prostate cancer.