Rapid and highly sensitive detection of DNA is critical in diagnosing genetic diseases. Conventional approaches often rely on cumbersome, semi-quantitative amplification of target DNA to improve detection sensitivity. In addition, most DNA detection systems ( microarrays, for example), regardless of their need for target amplification, require separation of unhybridized DNA strands from hybridized stands immobilized on a solid substrate, and are thereby complicated by solution-surface binding kinetics(1,2). Here, we report an ultrasensitive nanosensor based on fluorescence resonance energy transfer ( FRET) capable of detecting low concentrations of DNA in a separation-free format. This system uses quantum dots ( QDs)(3-5) linked to DNA probes to capture DNA targets. The target strand binds to a dye-labelled reporter strand thus forming a FRET donor - acceptor ensemble. The QD also functions as a concentrator that amplifies the target signal by con. ning several targets in a nanoscale domain. Unbound nanosensors produce near-zero background fluorescence, but on binding to even a small amount of target DNA (similar to 50 copies or less)they generate a very distinct FRET signal. A nanosensor-based oligonucleotideligation assay has been demonstrated to successfully detect a point mutation(6) typical of some ovarian tumours in clinical samples.