The generation of panels of mutant mice by homologous recombination has greatly increased the ability to assess the function of particular gene products in vivo, The ability to control the developmental stage, the tissue and the nature of the mutation would be an important innovation, A recent report((1)) demonstrates that the conservative site-specific recombination of bacteriophage pi, namely Cre-lox, can be used successfully in combination with homologous recombination to generate temporal- and cell-restricted mutations in vivo, This technical advance allows a greater flexibility in gene targeting and will have a significant impact on how complex gene functions are studied in vivo.