Defects in DNA damage response or repair mechanisms during neurogenesis result in genomic instability, which is causative for several neural defects. These include brain tumors, particularly medullo-blastoma, which occurs in the cerebellum with a high incidence in children. We generated an animal model with defective base excision repair during brain development through selective inactivation of DNA polymerase beta (Polb) in neuroprogenitor cells. All of Polb conditional knockout mice developed medulloblastoma in a p53 null background, similar to the Xrcc1 and p53 double deficient animal model. XRCC1 is a scaffolding protein which is involved in DNA damage repair and binds to POLB. In both animal models, the histopathological characteristics of the medulloblastoma were similar to those of human classic medulloblastoma. Brain tumor development was slower in the Polb and p53 double null animals than in the Xrcc1 and p53 double knockout animals. Molecular marker analysis suggested that Polb- and Xrcc1-deficient medulloblastomas belonged to the SHF alpha subtype, underscoring the important role of genomic stability in preventing this devastating pediatric cerebellar tumor. (C) 2018 The Authors. Published by Elsevier Inc.