PhK deficiency is very complex and many genes play a key role in the GSD IX phenotype; the spectrum includes autosomal recessive forms of GSD IX (caused by mutations in PHKB, PHKG1, PHKG2, CALM1, CALM2, and CALM3) as well as the X-linked liver form (caused by mutations in PHKA2) and X-linked muscle form (caused by mutations in PHKA1). The gene discussed is PHKA2; the disease is hyperinsulinemic hypoglycemia, familial, 4.