The hallmark characteristics of CMS are accentuated hypoxemia, excessive erythrocytosis (women: Hb ≥ 190 g/L, men: Hb ≥ 210 g/L), and pulmonary hypertension, which often results in right ventricular enlargement and hypertrophy, and in the later disease stage, leads to chronic heart failure. This evidence concerns the gene GSTM1 and pulmonary arterial hypertension.