Non-radioactive in situ hybridization (ISH) is a relatively new and sensitive method for analysis of chromosome aberrations. ISH may be performed on metaphase spreads or directly on interphase cells. By means of ISH, the so-called probes are bound to well-defined regions on the chromosomes of a cell. After a visualization process the probes and thus indirectly the chromosome regions may be observed in a microscope. Chromosome-specific probes become bound to chromosomal DNA segments which are specific for certain groups of chromosomes or only for a single chromosome. The chromosome-specific probes may be subdivided into 1) repetitive probes which are bound at or around the centromere, 2) painting probes which are bound to a certain chromosome in its entire extent or to a limited segment of this and 3) locus-specific probes which are bound to a single unique DNA sequence in the genome. ISH with chromosome-specific probes is of great scientific significance for chromosomal localization of genes. It is to be anticipated that this technique may be employed clinically e.g. in prenatal diagnosis and in the diagnosis of cancer and viruses.