In this study, we have examined the effects of targeting vector configuration and site of vector linearization on the frequency of targeted recombination at the endogenous CHO APRT locus, and have analyzed the types and class distributions of APRT+ recombinants obtained in APRT targeting experiments employing uncut circular, insertion-type (ends-in), and replacement-type (ends-out) configurations of the same pAG7 targeting vector, including configurations produced by introduction of a double-strand break (DSB) at sites either within, or at the 5' or 3' boundaries of APRT targeting homology. Our results suggest that: 1) plasmid-chromosome targeted recombination in mammalian cells may not be stimulated to the same degree by a DSB in the targeting vector as by a DSB in the chromosomal target; 2) recombinant class distributions are highly dependent upon targeting vector configuration; and 3) one-sided invasion mechanisms may play a significant role in homologous recombination in mammalian cells.