The homozygous mutation G53.385.643c379C > T in the GCLC gene resulting in the amino acid change pArg127Cys (arginine to cysteine at position 127) is very rare (0.0009% in the general population) and indeed consistent with gamma-glutamylcysteine synthetase deficiency. The gene discussed is GCLC; the disease is hyperinsulinemic hypoglycemia, familial, 4.