TGFB1 and papilloma: Overexpression of TGFβ1 in the epidermis blocks TPA-induced hyperplasia and papilloma formation [90] and Tgfb1−/− keratinocytes transduced with a v-RasHa retrovirus rapidly form SCC in athymic mouse skin grafts, while Tgfb1+/+ keratinocytes develop only benign papillomas [48].