PHKA2 and disorder of glycogen metabolism: Analysis of the patient's genetic sequences revealed a mutation known to cause GSD; the cytosine at nucleotide position 133 in the PHKA2 gene on the X-chromosome was replaced by thymidine (c.133C>T), resulting in a change from arginine (Arg) to tryptophan (Trp) at amino acid position 45 in the PhK protein.