During the last two decades, techniques for the detection of specific DNA and RNA sequences in situ have been developed. In the beginning, only radioactive detection was possible, which to some extent made these techniques unsuited for routine purposes. Within the last decade, a number of nonradioactive detection methods have become available, resulting in a much more widespread use of in situ hybridization. These methods have become of great value, e.g., as a supplement to traditional cytogenetic analysis in the identification and quantification of numerical and/or structural aberrations in both clinical genetics and cancer diagnosis.