The colored chromosomes in this illustration are the product of a novel labeling technique called multiplex fluorescence in situ hybridization, or mfish. Traditional chromosome staining produces black- and-white banding patterns. mfish, though, uses colored fluorescent tags, either singly or in combination, attached to fragments of single-stranded human chromosomes--a different color combination for each of the 24 chromosomes. The chromosomes here were isolated from cells, heated until each DNA double helix unzipped into two complementary strands, and then mixed with previously prepared, tagged DNA fragments. The fragments clung to their opposite partners, closing the zipper once more, but this time causing it to glow brightly when zapped with light. The technique enables its inventors--Michael Speicher of Yale and colleagues--to pick out abnormalities traditional methods might miss, as when chunks of DNA break away from their home chromosomes and reattach to others. That can cause such diseases as mental retardation and leukemia--and it changes the mfish color pattern.