Improved reporter strain for monitoring Cre recombinase-mediated DNA excisions in mice

Effective use of conditional Cre recombinase-loxP gene modification require s Cre-expressing mouse strains with defined patterns of expression. To asse ss the in vivo functionality of Cre expressing mice, we have engineered an improved reporter strain for monitoring Cre-mediated excisions. The beta-ga lactosidase-neomycin phosphotransferase fusion gene (beta geo)-trapped ROSA 26 locus was modified by gene targeting such that beta geo is expressed onl y after Cre-mediated excision of loxP-flanked DNA sequences. beta geo from the excised ROSA26 allele is expressed ubiquitously in embryos and adult mi ce. By mating the reporter strain with Cre-expressing transgenic mice, we h ave shown that the loxP-flanked ROSA26 allele is accessible to Cre during e arly embryogenesis, as well as in a specific hematopoietic lineage (T lymph ocytes). This improved reporter strain should facilitate monitoring in vivo Cre-mediated excision events in a variety of experimental contexts.