We have observed that deletion of either Rbl1 or E2f1 gene in a background of inducible epidermal loss of Rb1 (Rbf/f;K14creERTM;p107−/− or Rbf/f;K14creERTM;E2F1−/− mice, respectively) leads to aggravation of the phenotype caused by Rb1 loss (observed both in Rbf/f;K14cre and in Rbf/f;K14creERTM mice) allowing spontaneous tumor development [10, 13]. This evidence concerns the gene E2F1 and neoplasm.