Targeted homologous recombination at the endogenous adenine phosphoribosyltransferase locus in Chinese hamster cells.

We have developed a system that permits analysis of targeted homologous recombination at an endogenous, chromosomal gene locus in cultured mammalian cells. Using a hemizygous, adenine phosphoribosyltransferase (APRT)-deficient, Chinese hamster ovary (CHO) cell mutant as a transfection recipient, we have demonstrated correction of a nonrevertible deletion mutation by targeted homologous recombination. Transfection with a plasmid carrying a fragment of the APRT gene yielded APRT+ recombinants at a frequency of approximately 4.1 x 10(-7). The ratio of targeted recombination to nontargeted integrations of plasmid sequences was approximately 1:4000. Analysis of 31 independent APRT+ recombinants revealed conversions of the endogenous APRT gene, targeted integration at the APRT locus, and a third class of events in which the plasmid donor APRT fragment was converted to a full-length, functional gene.