Evaluation of recombination efficiency. (a) Schematic experimental strategy. Four ES cell lines carrying a single copy of CAG promoter-lox71-bsr-pA were established and coelectroporated with an integration plasmid and the Cre expression vector, pCAGGS-Cre. After G418 selection, the colonies were stained with X-gal, and the percentage of blue colonies was scored as the site-specific integration efficiency. (b) Example of X-gal staining. Bs17 ES cells were coelectroporated with pCAGGS-Cre and pKR3NZneo, selected with G418 for 1 week, and then stained with X-gal. (c) Magnified photo of blue colonies. Most of blue colonies were stained uniformly. (d) X-gal stained plate electroporated without pCAGGS-Cre. Bs17 ES cells were electroporated with only pKR3NZneo. No blue colony appeared.