Glycogen storage disease type IXα1 (GSD IXα1, historically referred to as GSD IXd), is an ultra-rare X-linked recessive inherited disorder of carbohydrate metabolism caused by pathogenic variants in the PHKA1 gene (OMIM: 300559, ORPHA: 715, ICD-11: 5C51.3, ICIMD: Muscle phosphorylase kinase subunit alpha 1 deficiency). Here, PHKA1 is linked to hyperinsulinemic hypoglycemia, familial, 4.