A congenital myasthenic syndrome characterized by autosomal dominant inheritance of presynaptic neuromuscular junction defects, early-onset muscle weakness, easy fatigability, delayed psychomotor development and ataxia that has_material_basis_in heterozygous mutation in the SNAP25 gene on chromosome 20p11. [ http://purl.obolibrary.org/obo/ECO_0007645 ]