KRAS and papilloma: We anticipated that localized activation of the Cre:ER recombinase by topical application of 4 hydoxytamoxifen (4HT) would result in the genetic recombination event that would remove the transcriptional stop sequence, resulting in expression of the K-Ras mutant G12D allele and ultimately leading to Ras-driven papillomas that would progress to form epidermal carcinomas.