Oncogene-induced senescence is a cellular response that may be crucial for protection
against cancer development,, but its investigation has so far been restricted to cultured cells
that have been manipulated to overexpress an oncogene. Here we analyse tumours
initiated by an endogenous oncogene, ras, and show that senescent cells exist in
premalignant tumours but not in malignant ones. Senescence is therefore a defining feature
of premalignant tumours that could prove valuable in the diagnosis and prognosis of cancer.