hemoglobin S (Hb S), hemoglobinopathy characterized by the substitution of the amino acid valine for glutamic acid at position 6 in the beta chain of the hemoglobin molecule. Hemoglobin S migrates to between hemoglobin C and hemoglobin A in hemoglobin electrophoresis. As the hemoglobin S becomes deoxygenated, red cells become sickle-shaped, occluding capillaries. If the proportion of Hb S to Hb A is large, as in sickle cell anemia, local infarction occurs. See also sickle cell anemia, sickle cell crisis, sickle cell trait.
