MAPK3 and skin basal cell carcinoma: Although BCC in both the Ptchflox/flox ERT2+/− and Ptchflox/flox K5-Cre-ERT model are identical based on histology and also at the level of Hh signaling activity (i.e., BCC of both models express Gli1 and Ptch), K5-Cre-ERT is highly leaky resulting in BCC formation even without Cre activation.