ATG9A and hyperinsulinemic hypoglycemia, familial, 4: In addition, fibroblasts from an individual with a heterozygous loss-of-function mutation in AP4E1 (the phenotypically normal mother of the homozygous AP4E1 patient) displayed normal ATG9A localisation, so mislocalisation of ATG9A is a cellular phenotype that correlates with disease in AP-4 deficiency.