In hemizygous males and homozygous females, G6PD deficiency is fully expressed, whereas in female heterozygotes, a mixed population of normal and enzyme-deficient cells can be found, owing to random inactivation of one of the two X chromosomes early in embryonic life (Lyonization) [3]. This evidence concerns the gene G6PD and hyperinsulinemic hypoglycemia, familial, 4.