Conditional DNA excision between two LoxP sites can be achieved in the mouse using Cre-ER(T), a fusion protein between a mutated ligand binding domain of the human estrogen receptor (ER) and the Cre recombinase, the activity of which can be induced by 4-hydroxy-tamoxifen (OHT), but not natural ER ligands. We have recently characterized a new ligand-dependent recombinase, Cre-ER(T2), which was approximately 4-fold more efficiently induced by OHT than Cre-ER(T) in cultured cells. In order to compare the in vivo efficiency of these two ligand-inducible recombinases to generate temporally-controlled somatic mutations, we have engineered transgenic mice expressing a LoxP-flanked (floxed) transgene reporter and either Cre-ER(T) or Cre-ER(T2) under the control of the bovine keratin 5 promoter that is specifically active in the epidermis basal cell layer. No background recombinase activity could be detected, while recombination was induced in basal keratinocytes upon OHT administration. Interestingly, a dose-response study showed that Cre-ER(T2) was approximately 10-fold more sensitive to OHT induction than Cre-ER(T).