A vector containing a CAG promoter driving expression of a floxed neo/STOP cassette, frt flanked lacZ gene with 3 polyadenylation sequences (secondary STOP cassette), and an EGFP gene was inserted into the locus. Germ line, flp mediated recombination removes the lacZ/STOP cassette leaving the neo/STOP cassette intact. Cre mediated recombination is necessary to achieve EGFP expression.