The actual pointed coat coloring that Siamese and other breeds are known for (as well as rats, mice, and rabbits) is simply a recessive form of partial albinism called "Himalayan". The Himalayan mutation alters the enzyme responsible for melanin production, which is called Tyrosinase. The mutation restricts the ability of tyrosinase to oxidize tyrosine only at cooler temperatures; warmer temperatures deactivate the enzyme.