A targeting vector was designed from the original gene trap strain, ROSA beta-geo26, to include a splice acceptor sequence (SA), a neo expression cassette flanked by loxP sites, a lacZ gene, and a polyadenylation (bpA) sequence inserted at a unique Xba1 site approximately 300-bp 5' of the original gene-trap integration site. In addition, a triple polyadenylation sequence was added to the 3' end of the neo expression cassette to prevent transcriptional read-through. Presence of the floxed neo cassette prevents lacZ expression. When crossed with a cre transgenic strain, lacZ is expressed in all cells/tissue where Cre is expressed.