Hemoglobin S: The most common type of abnormal hemoglobin and the basis of sickle cell trait and sickle cell anemia.
Hemoglobin S differs from normal adult hemoglobin (called hemoglobin A) only by a single amino acid substitution (a valine replacing a glutamine in the 6th position of the beta chain of globin). Recognition of this tiny change in the hemoglobin molecule marked the opening of molecular medicine.
