A rare, genetic hemoglobinopathy characterized by anemia, reticulocytosis and erythrocyte abnormalities including target cells, irreversibly sickled cells and crystal-containing cells. Clinical course is similar to sickle cell disease, but less severe and with less complications. Signs and symptoms may include acute episodes of pain, splenic infarction and splenic sequestration crisis, acute chest syndrome, focal segmental glomerulosclerosis, ischemic brain injury, peripheral retinopathy, and osteonecrosis.