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.