The p53-binding protein 53BP1 has been implicated in the DNA damage response and genomic instability. Previous reports have highlighted these roles in vivo in haematopoietic lineages. To investigate the importance of 53BP1 to the DNA damage response in epithelial cells in vivo, we have investigated the role of 53BP1 in mediating apoptosis and proliferation within the murine small intestine following γ-irradiation. 53BP1 deficiency does not affect the immediate response to γ-irradiation with normal levels of apoptosis, proliferation and p53 and p21 accumulation. However, 48 h post-γ-irradiation there was a significant accumulation of cells with much larger nuclei marked by p53 and p21 accumulation. These data reflect increases in polyploidy observed 53BP1−/− deficient fibroblasts following γ-irradiation. At 72 h post-irradiation both the 4N and 8N populations were significantly increased in 53BP1−/− MEFS. Taken together, these results show that following in vivo exposure to γ-irradiation, 53BP1 is dispensable for signalling apoptosis and cell-cycle arrest in the intestinal epithelium. However, it is important for prevention of genomic instability within this epithelial cell population.