Glycogen phosphorylase V, McArdle's Disease). More than 65 mutations in the PYGM gene that lead to McArdle disease have been identified to date. Symptoms of McArdle disease include muscle weakness, myalgia, and lack of endurance, all stemming from low glucose levels in muscle tissue. Mutations in the liver isoform of glycogen phosphorylase (PYGL) are associated with Hers' Disease (glycogen storage disease type VI). Hers' disease is often associated with mild symptoms normally limited to hypoglycemia, and is sometimes difficult to diagnose due to residual enzyme activity. The brain isoform of glycogen phosphorylase (PYGB) has been proposed as a biomarker for gastric cancer.
Glycogen-branching enzyme deficiency caused by an autosomal recessive mutation to the GBE1 gene, which leads glycogen branching enzyme activity that is reduced to absent. Subsequently, glycogen molecules are produced with few branches, which greatly decreasing the number of nonreducing ends, drastically slowing the rate at which the molecule can be synthesized or broken down. This causes low levels of muscle glycogen that is very resistant to amylase. Lacking proper glycogen storage, the horse's brain, heart muscle, and skeletal muscles cannot function, leading to rapid death. Most foals with glycogen-branching enzyme deficiency (GBED) are aborted (usually in the second trimester), or are stillborn, and