In a paper published online in advance in PNAS this week, researchers at Mount Sinai School of Medicine in New York and the University of Illinois at Urbana-Champaign present "an experimental approach based on multicolor single-molecule fluorescent in situ hybridization to study the composition of viral RNAs at single-virus particle resolution." The team applied its FISH-based approach to determine the copy number of each RNA segment within individual virus particles for th