GSTM1 and congenital myasthenic syndrome: CMS is a progressive incapacitating syndrome characterized by EE (Hb concentration ≥21 g/dL in men and ≥19 g/dL in women), severe hypoxemia, and by signs and symptoms such as headache, dizziness, breathlessness and/or palpitations, sleep disturbances, physical and mental fatigue, distended veins, and localized cyanosis (León-Velarde et al., 2005).