Trinucleotide repeat expansions in genomic DNA are the molecular basis of a number of genetic diseases. The (CAG)n, (CTG)n, and (CGG)n repeats share the common sequence CXG, and inherently produce a hairpin structure involving X-X mismatch base pairs flanked by two G-C base pairs. The chance of a strand slippage leading to the repeat expansion is considered to increase with increasing stability of the hairpin form. Here we show our synthetic ligand naphthyridine-azaquinolone (NA) stabilized a hairpin form of the (CAG)n repeat and inhibited the polymerase-mediated DNA synthesis.