Testosterone therapy should not be started in men with breast or prostate cancer, palpable prostate nodule or induration, and prostate-specific antigen (PSA) > 4 ng/mL or > 3 ng/mL in those at increased risk of prostate cancer (e.g., men with a first-degree relative with diagnosed prostate cancer) without further urological evaluation, elevated hematocrit, untreated severe obstructive sleep apnea, severe lower urinary tract symptoms, uncontrolled heart failure, myocardial infarction or stroke within the last 6 months, or thrombophilia. The gene discussed is KLK3; the disease is heart failure.