G6PD deficiency is caused by mutations in the G6PD gene on chromosome X. Genetically, males are either G6PD deficient or G6PD normal, while females can be homozygous with G6PD deficiency (mutations are present on both X chromosomes) or heterozygous (one X chromosome is affected) or G6PD normal. This evidence concerns the gene G6PD and hyperinsulinemic hypoglycemia, familial, 4.