Knock-In Animals A ???knock-in animal??? is a genetically modified animal (e.g., a mammal such as a mouse or a rat) in which an endogenous gene has been substituted in part or in total with a heterologous gene (i.e., a gene that is not endogenous to the locus in question; see Roamer et al., New Biol. 1991, 3:331), an orthologous gene from another species, or a mutated gene.