Sickle cell disease is a blood disorder that causes the bone marrow to produce red blood cells with defective hemoglobin (hemoglobin S). Hemoglobin S causes red blood cells to become misshaped, or sickle-shaped, when they release oxygen to other tissues in the body. Sickled red blood cells are destroyed faster than normal red blood cells. Rapidly destroyed red blood cells form a condition known as hemolytic anemia.