We have introduced a specific point mutation into the gene for chromosomal elongation factor 2 (EF-2) in Chinese hamster ovary cells (CHO-K1) by in vivo homologous recombination. To obtain a selectable construct for gene-targeting, we modified a diphtheria toxin-resistant mutant EF-2 gene (Gly717 to Arg) by deleting its promoter and first exon so that homologous recombinants could be distinguished from randomly integrated transformants, and also by inserting a second positive selection marker, the neomycin-resistance gene, into the 3'-flanking region to increase the selective accuracy. More than 80% of the clones surviving after selection for resistance to both the toxin and neomycin were the desired homologous recombinants in which the wild type, toxin-sensitive EF-2 gene was replaced by the modified gene giving resistance to both the toxin and neomycin. This result shows that the specific point mutation was co-introduced with a second selective marker into an endogenous chromosomal gene and that the modified gene was expressed.