Sickle cell anemia is one of the best studied inherited diseases, and despite being caused by a single point mutation in the HBB gene, multiple pleiotropic effects of the abnormal hemoglobin S production range from vaso-occlusive crisis, stroke, and pulmonary hypertension to osteonecrosis and leg ulcers. The gene discussed is HBB; the disease is pulmonary arterial hypertension.