Base Excision Repair (BER) pathway is correlated with nucleotide repeat instability. In this report, we investigated the modulatory effects of a DNA-binding small molecule, naphthyridine azaquinolone (NA), towards BER in an in vitro system. Thermal melting analyses demonstrated binding of NA to deaminated 5'-CAG-3'/5'-CAG-3' triads in DNA. Furthermore, binding of NA to the deaminated CAG repeat hairpin was found to partially inhibit UNG2- and APE1-catalyzed reaction, suggesting a potential mechanism for NA-induced CAG repeat contraction via BER pathway.