A novel suspension hybridization 
assay was used to determine 
nucleic acid copy number by 
flow cytometry. The 
assay was validated with low copy (lc) products 
ranging in length from 100 to 2304 bp conjugated to spectrally-distinct 
polystyrene microspheres. In the example provided herein, these conjugated microspheres were used as 
multiplex hybridization probes to detect 
homologous sequences in 
genomic DNA extracted from cytogenetic 
cell pellets and labeled with 
biotin-dUTP. Hybridization was detected with 
phycoerythrin-labeled 
streptavidin and analyzed by 
flow cytometry. Copy number differences were distinguishable by comparing the mean 
fluorescence intensities of test probes with a diploid reference probe in 
genomic DNA of patient samples and 
abnormal cell lines. The 
assay is capable of distinguishing a single 
allele and three alleles at a test locus from a biallelic reference sequence, regardless of chromosomal context. The assay is an improvement on previous methods which require prior amplification of locus-specific target 
DNA because, lc probes provide adequate specificity and sensitivity for accurate copy number determination of homologous targets. Because of its high sensitivity and accuracy, the assay is useful for determination of 
nucleic acid copy number for a variety of applications, including determination of genomic copy number in humans, animal models of 
disease and in solution, measurement of transcript levels, 
forensic DNA analysis, and 
quality control analysis in 
agriculture.