The maintenance of λ as a stable prophage within the chromosome of its lysogenic host requires the integrity of a specific region of the prophage called C I (Campbell, 1962), whose role is to repress, by means of a diffusible product, genes essential for the early vegetative development of the virus (Jacob and Monod, 1961). E. coli cells lysogenic for a λ prophage thus constitute a particularly suitable system for investigating in vivo (and possibly, in vitro) the features of repression at the DNA level. Not only does the early phage region under repressor command presumably comprise, unlike repressed bacterial operons, quite an appreciable portion of the total genome, but the “early” genes involved in phage replication happen to be clustered in a particular sector of the DNA molecule. This region, which approximately corresponds to the right half portion of the molecule, can be separated from the left portion (the...