Chromosomes of dividing cells are firstly stained. The resulting pattern of bands, similar to a barcode, is specific for a particular chromosome (1). The geneticist checks to see whether exactly two copies of the chromosome are present in each cell and whether the pattern of bands shows a correct sequence. In urgent cases chromosomes can be visualized by using dyes (2). This can be done both in dividing cells (image on the left) and in cells that are not dividing (image on the right). In both cases the cells of this fetus show three colored dots, suggesting the presence of trisomy. Comparative genomic hybridization (CGH) is a newly developed method that we use in selected cases where we suspect a genetic illness (3).