Yang et al. reported that of all reported cases of thiopurine drug toxicity in children with acute lymphoblastic leukemia (ALL), ∼20% can be explained by the TPMT activity and ∼22% by NUDT15 (Yang et al., 2015), while toxicity varies among ethnicities (e.g., NUDT15 polymorphism occurring more often in individuals of Asian descent). The gene discussed is TPMT; the disease is acute lymphoblastic leukemia.