Insertional mutagenesis represents a promising approach to the identification of new genes involved in mammalian development. In this paper, we have presented a brief review of the literature on the analysis of mutations caused by DNA and retroviral insertion into the mouse genome. We have discussed several methods that we and others have used to identify recessive insertional mutations among transgenic mouse lines. Finally, we have summarized the results of our studies to date on three recessive prenatal lethal mutations that we have identified.