Fluorescence in situ hybridization (FISH) is a powerful, molecular technique with a wide range of applications in medicine and biology. In medicine, FISH uses genomic and cDNA probes to determine the chromosomal position of genes and DNA sequences, which enables detection of ploidy levels and identification of subtle chromosomal rearrangements. Because of its exquisite sensitivity, FISH often enhances conventional cytogenetic analysis and it can provide either diagnostic or prognostic results for particular chromosomal disorders. To achieve the robust probe signals needed for routine clinical application, typical FISH probes consist of recombinant genomic DNA clones often covering multiple genes. These probes can be readily visualized on metaphase chromosomes as they span tens to hundreds of thousands of nucleotides. Visualization of shorter targets has been performed in many research laboratories, and will have significant advantages once they are available clinically. Toward that end, our goal is to make the signals from these shorter probes more intense. We are utilizing quantum dot conjugates to visualize single copy sequence DNA probes as short as 1500 nucleotides in length.