Phosphoglucomutase 3 (PGM3) deficiency (OMIM (Online Mendelian Inheritance in Man) #615816) is a rare autosomal recessive congenital disorder of glycosylation that affects the synthesis of UDP-GlcNAc, a key substrate in multiple glycosylation pathways [1]. The gene discussed is PGM3; the disease is hyperinsulinemic hypoglycemia, familial, 4.