Marker rescue. A. λ cI857 R-am5, bearing an amber mutation in the endolysin gene R, can form a plaque on the non-suppressing host only if it recombines with the plasmid or reverts. B. Efficiencies of mutants relative to wild type. Titers of the phage on cells bearing the homology-containing plasmid, minus titers on cells bearing the vector (revertants) are determined, and divided by the wild type value. Means and standard errors of four experiments are shown. All strains are recA.