Severe and sometimes fatal myelosuppression can occur in patients treated with thiopurine drugs who have complete TPMT deficiency due to homozygous or compound heterozygous TPMT*3A, TPMT*3B, TPMT*3C, or TPMT*2 genotypes [3]. Here, TPMT is linked to hyperinsulinemic hypoglycemia, familial, 4.