DNA polymerases synthesize new DNA during DNA replication and repair, and their ability to do so faithfully is essential to maintaining genomic integrity. DNA polymerase β (Pol β) functions in base excision repair to fill in single-nucleotide gaps, and variants of Pol β have been associated with cancer. Specifically, the E288K Pol β variant has been found in colon tumors and has been shown to display sequence-specific mutator activity. To probe the mechanism that may underlie E288K's loss of fidelity, a fluorescence resonance energy transfer system that utilizes a fluorophore on the fingers domain of Pol β and a quencher on the DNA substrate was employed. Our results show that E288K utilizes an overall mechanism similar to that of wild type (WT) Pol β when incorporating correct dNTP. However, when inserting the correct dNTP, E288K exhibits a faster rate of closing of the fingers domain combined with a slower rate of nucleotide release compared to those of WT Pol β. We also detect enzyme closure upon mixing with the incorrect dNTP for E288K but not WT Pol β. Taken together, our results suggest that E288K Pol β incorporates all dNTPs more readily than WT because of an inherent defect that results in rapid isomerization of dNTPs within its active site. Structural modeling implies that this inherent defect is due to interaction of E288K with DNA, resulting in a stable closed enzyme structure.