Ans. is 'a' i.e., Valine * Sickle cell disease is a common hereditary hemoglobinoopathy caused by a point mutation in b-globin that promotes the polymerization of deoxygenated hemoglobin, leading to red cell distortion, hemolytic anemia, micro- vascular obstruction, and ischemic tissue damage.* Several hundred different hemoglobinopathies caused by mutations in globin genes are known, but only those associated with sickle cell disease are prevalent enough in the United States to merit discussion.* Hemoglobin is a tetrameric protein composed of two pairs of globin chains, each with its own heme group.* Normal adult red cells contain mainly HbA (a2b2), along with small amounts of HbA2 (a2d2) and fetal hemoglobin (HbF; a2g2).* Sickle cell disease is caused by a point mutation in the sixth codon of b-globin that leads to the replacement of a glutamate residue with a valine residue.* The abnormal physiochemical properties of the resulting sickle hemoglobin (HbS) are responsible for the disease.