The cancer cell lines derived from Arf-null mice also had a normal DNA damage response (Fig. 5B, lanes 11–14), but in sharp contrast with those fibrosarcomas from wt or Atm-null mice, Arf-null fibrosarcomas conserved a functional p53/p21 response to DNA damage (lanes 11–14). The gene discussed is TP53; the disease is fibrosarcoma.