Glycogen storage disease type 7 (GSD7) is an inherited condition in which the body is unable to break down glycogen (a complex sugar) in the muscle cells. Because glycogen is an important source of energy, this can interfere with the normal functioning of muscle cells. The severity of the condition and the associated signs and symptoms vary, but may include muscle weakness and stiffness; painful muscle cramps; nausea and vomiting; and/or myoglobinuria (the presence of myoglobin in the urine) following moderate to strenuous exercise. Those who develop the condition during infancy may experience additional symptoms such as hypotonia (poor muscle tone), cardiomyopathy, and breathing difficulties. This condition is caused by changes in the PFKM gene and is inherited in an autosomal recessive manner.