Replacement vectors contain specific restriction sites flanking the nonessential genes. Digestion of linear vector DNA at these sites produces two "arms" that are ligated to the foreign DNA. Many commer­cially available X vectors are sold as predigested and purified arms. Replacement vec­tors typically can accommodate between 8 and 24 kb of foreign DNA, depending on the vector.

During the early phase of infection, X DNA replicates bidirectionally, in circular form from a single origin of replication before shifting to replication via a rolling-circle mode. This produces a concatamer of genomes in a head-to-tail arrangement that is then processed to give individual genomes for packaging. The shift to rolling-circle replication depends on the interplay between host- and phage-encoded recombi­nation functions. As the recombination proficiency of different X vectors can vary, the investigator is urged to ensure that the E. coli strain used for infection is capable of properly replicating the phage. This information is generally supplied with commer­cially available vectors. A great many features that aid cloning into and screening of recombinant phage have also been incorporated into X vectors. Often, the use of these features also necessitates the use of particular host strains.