Single-nucleotide polymorphisms (SNPs) are responsible for much of the genetic variations associated with susceptibility to various common diseases and response to various drugs. While a number of available methods for SNPs mapping and typing have been proposed based on hybridization or enzymatic technologies, 1-6 these methods require several time-consuming steps, the use of several kinds of fluorophore-labeled oligodeoxynucleotides, and/or special enzymes. A quick, simple and cost-effective method is therefore highly desirable for the routine detection of SNPs, which would allow for the implementation of tailor-made medication based on the genetic variations. 7 For this purpose, of our particular interest is the use of small ligands possessing hydrogen-bonding groups suitable for nucleobase recognition. [8][9][10][11][12][13][14][15][16][17][18] We have recently discovered a series of small aromatic ligands that can bind to a nucleobase opposite an abasic site (AP site) in DNA duplexes, and have proposed a new strategy of ligand-based fluorescence assay for SNPs typing. 11-18 Our method is based on the construction of the AP site in DNA duplexes, which allows small synthetic and/or biotic ligands to bind to target nucleotides, accompanied by fluorescence signaling: an AP site-containing probe DNA is hybridized with a target DNA so as to place the AP site toward a target nucleotide, by which hydrophobic microenvironments are provided for ligands to recognize target nucleobases. Highly selective bindings toward target nucleobases were indeed obtained with the binding affinity up to the micromolar range, as has been demonstrated in AMND (2-amino-7-methyl-1,8-naphthyridine)-cytosine, 11,13 pterin-guanine, 12 vitamin B2-thymine, 15,16 and amiloride-thymine 17 bindings. Our method was effectively applicable to the analysis of PCR (polymerase chain reaction) amplification products, 17 for which a complexation-induced fluorescence quenching of these AP sitebinding ligands was utilized to detect the single base mutation.However, in order to further assure the analysis based on fluorescence measurements, it is more desirable that ligands exhibit other types of sensing functions, such as a ratiometric emission response.Here, we report on a new class of AP site-binding fluorescence ligands, Naph-NBD (Scheme 1), for the ratiometric assay for SNPs typing. Our design strategy here is the introduction of another fluorophore to the mother skeleton via an appropriate linker, i.e., in addition to the blue fluorescent naphthyridine moiety suitable for cytosine recognition (emission maximum at 409 nm), 11,13 the ligand has the NBD (7-nitrobenzo-2-oxa-1,3-diazole) moiety, 19 which shows a green fluorescence (emission maximum at 544 nm). In this work, we investigate the binding of Naph-NBD to 11-mer AP sitecontaining duplexes (5â˛-CTG CCY CCA CC-3â˛/3â˛-GAC GGX GGT GG-5â˛, X = AP site; Spacer-C3, 18 Y = target), and demonstrate that Naph-NBD exhibits a ratiometric fluorescence response to detect pyrimidine/purine transversion.Naph-NBD w...