Expression of a tamoxifen-dependent cre-recombinase from a CAG-TBD-CRE-TBD transgenic construct electroporated into Runx1tm1Medv ES cells resulted in the excision of the previously inserted floxed sequence (Engrailed2-SA-lacZ), leaving a single loxP site within the 5'UTR of the locus. Northern blot analysis detected normal expression of major splice isoforms in lungs and thymus.