Fluorescence In-Situ Hybridization (FISH)
Fluorescence In-Situ Hybridization is a method used to identify specific parts of a chromosome. For example, if you know the sequence of a certain gene, but you don't know on which chromosome the gene is located, you can use FISH to identify the chromosome in question and the exact location of the gene. Or, if you suspect that there has been a translocation in a chromosome, you can use a probe that spans the site of breakage/translocation. If there has been no translocation at that point, you will se one signal, since the probe hybridizes to one place on the chromosome. If, however, there has been a translocation, you will see two signals, since the probe can hybridize to both ends of the translocation point.
To use FISH efficiently, you have to know what you're looking for, i.e. you usually suspect a particular defect, based on the appearance of certain chromosomes, etc.
Here's how it works: