TP53 and fibrosarcoma: The large majority of 3MC-fibrosarcomas produced in Atm-wt mice (19/22 = 86%, after combining p53-wt and p53-super) had an immunohistochemical profile consistent a “p53 mutant” pattern (i.e. p53-strongly positive & p21-negative; Table 1, see example in Fig. 5A), thus confirming the strong pressure to inactivate p53 in this cancer model.