PTTG is a multifunctional proto-oncogene overexpressed in thyroid cancers, which binds to p53 and modulates its function. PBF, a binding partner of PTTG, is also overexpressed in thyroid cancer and can transform cells independently of PTTG. Moreover, subcutaneous expression of PBF elicits large tumours in nude mice. Given the established role of ionising radiation in thyroid tumourigenesis, we investigated the relationship between PBF and the tumour suppressor protein p53. PBF repressed p53-mediated gene regulation through HDM2 promoter assays in p53-null H1299 cells. Transfection of p53 elicited a 143±17-fold stimulation of promoter activity, whereas co-transfection of PBF significantly repressed p53 transcriptional activity (41±5-fold, P<0.001). Exposure of wild-type mouse thyrocytes to gamma-irradiation resulted in a significant increase in PBF protein expression after 24 h (1.88±0.09-fold, P<0.017). Co-immunoprecipitation assays revealed direct binding of PBF and p53 in TPC-1 human thyroid papillary carcinoma cells, with a marked increase in binding after treatment with gamma-irradiation. Furthermore, transient overexpression of PBF in TPC-1 cells resulted in a significant decrease in p53 protein levels compared to controls (75±2.5% decrease after 90 min, P<0.028, n=4). Finally in MTT proliferation assays, we observed a significant reduction of cell viability in mock-transfected TPC-1 cells after treatment with gamma-irradiation compared to untreated controls (17.9±0.008% decrease, P<0.016, n=4). Critically, overexpression of PBF abrogated this observed decrease of cell viability (0.2±0.007%, P=NS, n=4). Taken together these data highlight a novel potential mechanism of thyroid tumourigenesis, whereby PBF stabilises in response to DNA damage, binds directly to p53 and inhibits its function.