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