MONDO_0012693 (glycogen storage disease due to muscle and heart glycogen synthase deficiency) is a glycogen storage disease characterized by muscle and heart glycogen deficiency. It has been described in three siblings (two brothers and their younger sister). The older brother died at 10.5 years of age as a result of sudden cardiac arrest and the younger brother presented with hypertrophic cardiomyopathy, abnormal heart rate and blood pressure during exercise, and muscle fatigability. The sister showed no symptoms but a lack of glycogen was identified through muscle biopsy. The syndrome is caused by homozygous missense mutations in the gene encoding muscle glycogen synthase. Also known as: GSD due to muscle and heart glycogen synthase deficiency, GSD type 0b, glycogen storage disease due to glycogen synthase deficiency of heart, glycogen storage disease type 0b, glycogenosis due to muscle and heart glycogen synthase deficiency, glycogenosis type 0b, heart glycogen storage disease due to glycogen synthase deficiency.