TP53 and lymphoma: However, while the overexpression of p53 during DN3 stage promotes a block in the differentiation and proliferation, resulting in a small thymus size,20,21 loss of p53 apparently does not affect thymocyte development, even though the vast majority of spontaneous malignancies in p53−/− mice are lymphomas.22 Thus, the double function of Numb could be dependent on two different pathways, which may be differentially triggered during selected differentiation stages.