Haptoglobin (HP), is a protein that in humans is encoded by the HP gene. Haptoglobin, a plasma glycoprotein that binds free hemoglobin, has a tetrameric structure of 2 alpha and 2 beta polypeptides that are covalently associated by disulfide bonds. Haptoglobin is homologous to serine proteases of the chymotrypsinogen family. In the chimpanzee, there are 3 genes in the haptoglobin family (haptoglobin, HP; haptoglobin-related, HPR; and haptoglobin-primate, HPP), whereas only 2 genes exist in humans (HP and HPR). There are similarities between the primary structures of the alpha chain and of light chains of gamma globulins. The alpha haptoglobin locus is on the long arm of chromosome 16, where its exact cytogenetic location is 16q22.2. A major function of haptoglobin is to bind hemoglobin (Hb) to form a stable Hp-Hb complex and thereby prevent Hb-induced oxidative tissue damage. Haptoglobin is an unusual secretory protein in that it is proteolytically processed in the endoplasmic reticulum and not in the Golgi. The human haptoglobin HP*2 allele contains a 1.7-kb intragenic duplication that arose after a unique nonhomologous recombination between the prototype HP*1 alleles.