MAPK3 and melanoma: To test whether developing tumors would be populated by pre-existing memory CD8 + T cells, OT-I chimeras were inoculated with 1.5 × 105 aggressively growing B16 melanoma cells i.d. Alternatively, OT-I chimeras were generated in BRafCA,PtenloxP,Tyr::Cre-ERT2 mice (herein referred to as Braf/Pten), which develop local autochthonous tumors in skin after topical application of tamoxifen through Cre-mediated deletion of the tumor suppressor Pten and expression of the mutant BrafV600E oncogene in melanocytes10.