The potential now exists in many experimental systems to transfer a cloned, modified gene back into the genome of the host organism. In the ideal situation, the cloned gene is returned to its homologous location in the genome and becomes inserted at the target locus. This process is a controlled means for the repair of DNA damage and ensures accurate chromosome disjunction during meiosis. The paradigm for thinking about the mechanism of this p- cess has emerged primarily from two sources: (1) The principles of reaction mechanics have come from detailed biochemical analyses of the RecA protein...