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 a nuclear localization signal, and an EGFP gene was inserted into the locus. Germ line, cre mediated removal of the neo/STOP cassette allows for ubiquitous expression of the lacZ gene. The STOP cassette following the lacZ gene remains intact and prevents the expression of EGFP. Expression of EGFP requires flp mediated removal of the secondary STOP cassette.