Schematic representation of the evolution of the hp gene cluster and the origin of the hpr gene in primates. The hpr gene arose by a triplication of the hp locus after the new world monkeys diverged from other primates. New world monkeys, such as the spider monkey (A), thus harbor only a single hp gene, whereas old world monkeys, exemplified by the chimpanzee (B), harbor an hp gene and an hpr gene, in addition to a so-called haptoglobin-primate (hpp) gene. In human evolution (C), one locus was deleted, leaving only 2 genes (hp and hpr) in the current “hp gene cluster.”