Recombination-deficient (rec−) mutants of E. coli define the Rec system, which promotes bacterial recombination (Clark and Margulies, 1965; Howard-Flanders and Theriot, 1966; Clark, 1967). Phage λ recombines essentially normally in rec− hosts, suggesting that there is an analogous λ-produced recombination system (Van de Putte et al., 1966; Takano, 1966; Brooks and Clark, 1967). This recombination system, the Red system, has now been defined by recombination-deficient (red−) mutants of λ (Echols and Gingery, 1968; Signer and Weil, 1968). In addition to the Red system, which promotes general recombination in vegetative phage crosses, λ also provides a site-specific integrative recombination system (Int) responsible for insertion of the viral DNA into that of the host (Weil and Signer, 1968; Signer and Weil, this volume; Echols et al., 1968).