are also references in literature calling white horses "albino". Dominant white in horses is caused by the absence of pigment cells (melanocytes), whereas albino animals have a normal distribution of melanocytes. In other animals, patches of unpigmented skin, hair, or eyes due to the lack of pigment cells (melanocytes) are called piebaldism, not albinism nor partial albinism. All so-called "albino" horses have pigmented eyes, generally brown or blue. In contrast, many albino mammals, such as mice or rabbits, typically have a white hair coat, unpigmented skin and reddish eyes. The definition of "albinism" varies depending on whether humans, other mammals, or other vertebrates are being discussed. Despite this, some registries still refer to "albino" horses. For example, the Paso Fino Horse Association registers cremellos and other cream colors as "albino." Until 1999, the American Quarter Horse Association (AQHA) described perlino or cremello horses as "albino" in rule 227(j). The AQHA later replaced the word "albino" with "cremello or perlino," and in 2002 the rule was removed entirely. Among Connemara pony breeders, homozygous creams are called "blue-eyed creams" or sometimes "pseudo-albino". Types of albinism in humans and other animals The best-known type of albinism is OCA1A, which impairs tyrosinase production. In other mammals, the diagnosis of albinism is based on the impairment of tyrosinase production through defects in the Color (C) gene. Mice and other mammals without tyrosinase have unpigmented pink skin, unpigmented white hair, unpigmented reddish eyes, and some form of vision impairment. No mutations of the tyrosinase or C gene are known in horses. Humans exhibit a wide range of pigmentation levels as a species. However, the diagnosis of albinism in humans is based on visual impairment, which has not been described in white horses. Vision problems are not associated with gray, dilute, or white coat colors in horses, and blue eyes in horses do not indicate poor vision. Eyes are pigmented at the front of the iris called the stroma, and in a thin layer at the back of the iris in tissue called the iris pigment epithelium. The iris pigment epithelium prevents damaging light scattering within the eye. Blue-eyed humans and mammals have little or no pigment in the stroma, but retain pigment in the iris pigment epithelium. If pigment is missing from both the stroma and the iris pigment epithelium, the only pigment in the eye is the hemoglobin in blood vessels. This accounts for the