TPMT deficiency (1 in 300 individuals) is associated with bone marrow failure when such patients are treated with standard doses of thiopurine drugs (Lennard et al, 1989; Evans et al, 1991; McBride et al, 2000); a less severe myelosuppression can develop in TPMT heterozygotes (‘intermediate’ activity, 11% of subjects), (Weinshilboum & Sladek, 1980; Lennard et al, 1990; Relling et al, 1999a). Here, TPMT is linked to hyperinsulinemic hypoglycemia, familial, 4.