Red-mediated chromosomal gene replacement. A. Strains bearing an alteration of the lac locus in which lacI through the N-terminal coding region of lacZ is replaced by the C-terminal two-thirds of the bla gene and the aph gene of Tn903 can recombine with a linear dsDNA generated by PCR and introduced by electroporation, creating an ampicillin-resistant, Lac+ recombinant. B. Efficiencies of mutant strains relative to wild type. Means and standard errors of three or four experiments are shown.