The p57-E2F1-p53 pathway in thymocyte development and lymphomagenesis. p57 regulates E2F1 to control E2F target gene expression and p53 activity during normal thymocyte development. In the absence of p57, increased E2F target expression and p53 hyperactivation contribute to the arrest of thymocyte development at the DN3 to DN4 transition. The developmental arrest of thymocytes lacking p57 is partially rescued by the loss of E2F1. Loss of p53 in thymocytes lacking p57 leads to acceleration of thymic lymphoma development, suggesting that the observed p53 hyperactivation in the absence of p57 executes an important tumor suppressor function in thymocytes. Professional illustration by Marie Dauenheimer.