Medullary thyroid cancer penetrance was 25.0% (95% CI, 0.6%-80.5%) in high-risk RET variant carriers (1 of 4) and 12.3% (95% CI, 5.8%-22.1%) in moderate-risk carriers (9 of 73 carriers) (eTables 5 and 6 in Supplement 1). This evidence concerns the gene RET and medullary thyroid gland carcinoma.