A targeting construct was designed to insert sequence encoding IRES, floxed neo, and the SV40 early region in the 3' untranslated region of beta actin, 55 bp downstream of the stop codon. The sequence encoding the SV40 early region encoded large T, small t, and 17kT. The inserted cassette leaves the endogenous coding region and the 54 bp 'zipcode' that directs cellular localization of beta actin intact. Tag expression is dependent on the removal of the floxed neo cassette (which occurred at low frequency without cre expression).