A site 5' of the Ubb gene was targeted with a construct composed of a CMV/beta-actin enhancer and promoter (CAG), a floxed beta-geo with three transcription termination sites and an HIST1H2BB and GFP (H2B-GFP) fusion. Cre-mediated excision of the floxed beta-geo construct is required to achieve expression of the H2B-GFP cassette. (J:193332)