A neuromuscular junction disease that is characterized by weakness and easy fatiguability resulting from a genetic defect at the junction where the nerve stimulates muscle activity that result in muscle weakness and may affect nerve cells (presynaptic), muscle cells (postsynaptic) or the space between nerve and muscle cells (synaptic). [ http://purl.obolibrary.org/obo/ECO_0007636 http://purl.obolibrary.org/obo/ECO_0007638 ]