DNA Polymerase β (Pol β) is the main gap-filling polymerase involved in base excision repair (BER), a pathway responsible for the repair of small base lesions in the genome. Several germline amino acid substitution variants have been identified in the POLB gene, but little is known about their cellular and biochemical impact. In this study we tested the hypothesis that expression of the germline coding polymorphism P242R (rs3136797) in mammalian cells could induce a cancerous phenotype. Expression of P242R induced BER intermediates, genomic instability, and cellular transformation in both human and mouse cells. Biochemically, P242R had a decreased rate of catalysis than WT although P242R binds DNA with a similar affinity to WT. Our results suggest that people who carry this germline polymorphism may be at an increased risk for cancer susceptibility. In addition, patients who carry this polymorphism have a worsened response to chemotherapies suggesting that P242R may serve as a biomarker to predict survival.