The utility of tyrosinase minigene co-injection was evaluated as a visual marker for the generation and breeding of transgenic mice. In an evaluation of 39 transgenic founder animals and 44 transgenic lines five phenotypic patterns of pigmentation were consistently observed, including albino, dark, light, mottled and himalayan. In these studies co-Injection of the tyrosinase minigene along with the transgene of interest (TOI) resulted in genomlc integration of the two transgenes in 95% of the FO generation. Co-segregation of transgenes occurred in 94% of doubly transgenic mice In the F₁ generation, without dissociation in subsequent generations. All pigmented phenotypes proved useful for distinguishing homozygous from heterozygous F₂ animals via backcross trials, while the light, mottled and himalayan phenotypes proved useful in visually discriminating between homozygous and heterozygous F₂ animals. In addition, the light, mottled and himalayan phenotypes proved useful in determining segregation patterns of transgenes In the progeny of crosses between separate transgenic lines. Moreover, there appears to be a correlation between intensity of pigmentation and degree of expression of the co-injected TOI. These studies confirm that tyrosinase co-injection Is a useful adjunct in transgenic mouse studies and can serve to reduce routine genetic validation of transgenic lines.