Hemoglobin is composed of four polypeptide chains: two *-chains, each containing 141 amino acid residues. and two *chains, each containing 146 amino acid residues. Hemoglobin is located in red blood cells and performs the life-sustaining function of transporting oxygen from the lungs to the cells and carbon dioxide from the cells to the lungs. In some individuals, the hemoglobin molecules have a slightly different suquence of amino acid residues. It is so-celled abnormal human hemoglobin that have attracted particular attention because of the diseases associated with them. One of the best-known abnormal hemoglobins (designated HbS) differs from fhe normal type (HbA) by only one amino acid residue in each of the *-chains. In HbS, the glutamate in the sixth position of normal HbA is replaced by a valine residue: This difference affects only two position in molecule containing 574 amino acid residues, and yet it is enough to result in a very serious disease, sickle-cell disease.