reddish appearance of eyes in some types of albinism. In research mammals, such as mice, albinism is more strictly defined. Albino mice occur due to a recessive mutation of the C gene. No such mutation exists in horses. Albino mice lack pigment, but "...the inability of albino animals to produce pigment stems not from an absence of melanocytes...but from a deficiency and/or alteration of the structure of tyrosinase in melanocytes which are otherwise normal." This definition of albinism in mice – the inability to make tyrosinase – is extended to other mammals. While mammals derive their pigments only from melanins, fish, reptiles and birds rely on a number of pigments apart from melanins: carotenoids, porphyrins, psittacofulvins, pterins, etc. Most commonly, reptiles with a condition homologous to human OCA1A retain their reddish and orangish hues. As a result, birds and reptiles without the ability to manufacture tyrosinase are more accurately described as "amelanistic." Horses do not have non-melanin pigments and so if they were albino, would have no pigmentation. The retained pigment of dilute horses, like cremellos, is not comparable to the retained pigment of amelanistic "albino" birds and reptiles. Why cream is not albino The cream gene, which is responsible for palomino, buckskin, and cremello coat colors, was mapped to the MATP gene in 2003 (now known as SLC45A2). This gene is sometimes called the OCA4 gene, because one mutation on SLC45A2 is associated with Oculocutaneous albinism type 4. However, other mutations in SLC45A2 are responsible for normal variations in skin, hair, and eye color in humans and . Although SLC45A2 is not the "albino gene"; one of many mutations of the human SLC45A2 is responsible for a form of albinism . Famous white horses Many famous horses, past and present, were alleged to be "white" by observers, but were actually grays with hair coats turned fully white. Likewise, most white horses used in movies are actually grays, in part because they are easier to find. However, there are a few truly white horses who were used in film. One of the best-known examples was "Silver," ridden by the Lone Ranger, a role actually played by two different white horses. At least one horse who played "Topper," ridden by Hopalong Cassidy, was also white. Another famous white horse is Sodashi, a Japanese Thoroughbred racehorse who won Grade 1 races including Hanshin Juvenile Fillies, Oka Sho (Japanese 1,000 Guineas), and Victoria