Red blood cell hemoglobin is used to transport oxygen from the lungs to the tissues. It consists of two alpha chains and two beta chains. Hemoglobin S (Hb S) is an abnormal hemoglobin resulting from beta chain mutation. The gene responsible for beta chain synthesis is present in two copies on the chromosomes. Individuals with both copies of the mutated gene (homozygous) will have a high percentage of Hb S. Individuals with a single mutated copy (heterozygous or “sickle cell carriers”) will have a moderate level of Hb S (about 40%). Hb S causes “cell disease” or “sickle cell anemia,” a common genetic disease. Hb S causes the sickling of red blood cells (sickle formation) when oxygen levels are low in the blood (exercise, pulmonary infection, etc.). Sickle-shaped red blood cells can block small blood vessels, causing pain and a lack of tissue oxygenation. Abnormal blood cells are also more fragile, causing anemia. High levels of hemoglobin A or hemoglobin F may help limit sickle formation. Testing for the presence of Hb S is usually accompanied by a sickle cell test confirming the capacity of Hb S to form sickles.