Regarding Sos1/2 contribution to that signaling axis in the skin, it was also shown that expression of a dominant Sos1 transgene caused significant hyperproliferation of the basal keratinocyte compartment, leading to hyperplasia and development of skin papillomas with 100% penetrance, [25,26] and was also able to rescue the skin barrier defect caused by the absence of EGFR [27]. This evidence concerns the gene SOS1 and skin papilloma.