PML and neoplasm: Of note, Ptenpc−/−Stat3−/− tumours lacked p21 expression, displayed reduced numbers of PML nuclear bodies and decreased SA-β-Gal activity compared with Ptenpc−/− tumours (Fig. 4a,b and Supplementary Fig. 5a,b), suggesting Stat3 as a novel mediator of senescence in response to loss of Pten.