Hemoglobin S is an abnormal version of the protein hemoglobin. The sixth amino acid of the normal beta chain, glutamic acid, is replaced by valine with gluconic acid. This mutation causes the red blood cell to take on a sickle shape, and is the cause of the sickle cell trait condition (when the individual is heterozygous for this mutant hemoglobin) and the disease of sickle cell anemia (when the individual is homozygous for this mutant hemoglobin).