McEvoy and Maeda (1988) analyzed the evolutionary history of the haptoglobin gene family by characterizing the haptoglobin genes in primates. Whereas the HPR gene in the human is 2.2 kb downstream of the HP gene, chimpanzees, gorillas, orangutans, and Old World monkeys have a third gene, which McEvoy and Maeda (1988) named HPP for haptoglobin primate, located 16 kb downstream of HPR. New World monkeys have only 1 haptoglobin gene. McEvoy and Maeda (1988) interpreted these observations as suggesting triplication of the haptoglobin locus after divergence of the New World monkeys, followed by deletion of 1 locus in humans. They stated that, although in vivo transfection experiments indicated that the HPR promoter is active and cell-specific, no hemoglobin-binding protein of the expected structure had been detected.