A technique used to identify the presence of specific chromosomes or chromosomal regions. In clinical use, this is usually to detect those syndromes where submicroscopic pieces of DNA are missing - for instance Williams syndrome.
A fluorescently labelled single-stranded DNA sequence is used to bind with its complementary DNA sequence on a preparation of chromosomes. If the DNA sequence is present on the chromosomes of the patient, the fluorescent DNA sequence will bind, and can be visualised. If the sequence is absent, no signal will be seen.