Hemoglobin is the major component of erythrocytes, and is responsible for the red color. Its normal concentration in erythrocytes is approximately 34%. It is the most important respiratory protein of vertebrates by virtue of its ability to transport oxygen to tissues and carries carbon dioxide away from the tissues. Anomalous globins in which various amino acids have been substitued with others, or in which certain amino acids are missing entirely from the normal sequence, comprise 153 abnormal hemoglobin configurations. Sickle cell anemia is one such configuration. In sickle cell hemoglobin, a valine residue has replaced the glutamic acid residue at position 6 in the b-chain of normal hemoglobin; the a-chain is normal.