Normal result in the prenatal screening of Down syndrome: fluorescence Iin situ i hybridisation (FISH) micrograph of chromosomes from sampled cells of a normal fetus. FISH is being applied to prenatal detection of Down syndrome (Trisomy 21). Here, chromosome 21 DNA appears yellow when stained with fluorescein; the remaining cellular DNA appears red from staining with propidium iodide. Normal cells with 2 copies of chromosome 21 Uusually u show 2 yellow domains; Down syndrome cells have 3 copies of chromosome 21 & Uusually u show 3 yellow areas. Variance due to domain overlap is corrected by statistical methods.