A targeting vector was designed to insert floxed (flanked by loxP) phosphoglycerate kinase-1-cytosine deaminase/phosphoglycerate kinase-1-puromycin (PGK-CD/PGK-Puro) and stop sequences between the splicing acceptor (SA) and the beta-gal-neomycin phosphotransferase fusion gene (beta-geo) sequences of the proviral ROSA 26 locus. Correctly targeted, this resulted in ES cells that were Puro resistant but G418 susceptible, and did not express beta-gal. (J:64291)