Based on NUDT15 status, start with reduced starting doses (30%-80% of normal dose) if normal starting dose is ≥75 mg/m2/day or ≥ 1.5 mg/kg/day (e.g., start at 22.5-60 mg/m2/day or 0.45-1.2 mg/kg/day) and adjust doses of mercaptopurine based on degree of myelosuppression and disease-specific guidelines. Allow 2-4 weeks to reach steady-state after each dose adjustment. If myelosuppression occurs, and depending on other therapy, emphasis should be on reducing mercaptopurine over other agents (PMID 20354201, 18685564, 8857546, 18987654, 20010622, 16401827, 11302950, 16530532, 9634537). If normal starting dose is already < 75 mg/m2/day or < 1.5 mg/kg/day, dose reduction may not be recommended. TPMT phenotype could not be assigned based on genotyping performed. Consider evaluating TPMT erythrocyte activity to assess TPMT phenotype. If thiopurines are required and TPMT status is unknown, monitor closely for toxicity.