NPM-ALK induced senescence requires an intact p16INK4a/pRb pathway. Growth curves of (A) p16INK4a^−/− or (B) Rb1^−/− MEFs infected with the indicated retroviruses. Values are expressed relative to day 1. In each panel, results are representative of at least 3 independent experiments. *P values are relative to control cells infected with empty vectors. (C) Methylcellulose-CFU of WT or p16INK4a^−/− lin⁻ cells infected with the indicated retroviruses (graph and representative images). Values are expressed relative to control infected cells. (D) Serial replating of WT and p16INK4a^−/− lin⁻ cells infected with the indicated retroviruses. CFUs are expressed relative to the first count for each sample. Results are representative of 3 experiments. *P < .05 relative to control cells infected with an empty vector.