DNA polymerases play key roles in DNA synthesis by catalysing the polymerisation of deoxynucleotides opposite a parental (template) DNA strand to generate a new or repaired complementary daughter strand. Faithful replication by DNA polymerases is essential in maintaining genome integrity during cell division, DNA repair and DNA recombination. DNA polymerase beta (Pol β) plays a pivotal role in the base excision repair (BER) pathway by performing repair synthesis to fill single nucleotide gaps which arise during DNA repair. However, overexpression of Pol β is found in many human cancers and has been shown to promote a mutator phenotype. The aim of this study was therefore to investigate the effect of Pol β overexpression on BER. In vitro repair assays using whole cell extracts and DNA substrates containing site-specific BER lesions were conducted to compare two cell lines, one of which was derived from a cancer patient overexpressing Pol β. I found that overexpression of Pol β results in a 5 to 10-fold increased frequency of one nucleotide frameshift mutations and based on biochemical studies a mechanism is proposed to explain this phenomenon. I therefore conclude that an excess of Pol β can have potentially mutagenic consequences.