PYGL and disorder of glycogen metabolism: Similarly, Vockley et al. described a synergistic heterozygosity effect in patients with glycogen storage disorders (GSDs), consisting of two unrelated families with a clinical and biochemical diagnosis of GSDs involving simultaneous heterozygous variants in three different genes involved in the glycogen metabolic pathway, PYGL (GSD VI), AGL (GSD III), and GBE1 (GSD IV) [35].