Recombination during gene transfer into mouse cells can restore the function of deleted genes.

Two plasmids containing nonoverlapping deletions of the herpes simplex virus thymidine kinase gene were introduced into thymidine kinase-deficient mouse L cells by DNA-mediated gene transfer. Thymidine kinase-producing transformants were generated by a mixture of the two plasmids at a frequency significantly greater than that generated by either plasmid alone. Southern blot analyses demonstrated that functional thymidine kinase genes were generated by homologous recombination between the two deletion mutants.