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Fluorescence in situ hybridization is a laboratory test that looks for one specific, already-suspected genetic change, rather than scanning the whole genome for anything unusual. The technique works by using a short, custom-made piece of DNA, called a probe, that has been chemically labelled with a fluorescent tag. This probe is designed to match, and stick to, one exact region of one chromosome. When the probe is applied to a person’s cells on a laboratory slide, it binds only to its matching target, and a scientist then looks at the cells under a fluorescence microscope. If the expected region is present in the normal number of copies, the probe lights up in the expected pattern. If a piece of that region is missing, duplicated, or has moved to an unexpected location, the fluorescent pattern changes in a specific, recognizable way, revealing the imbalance.
Because this test is targeted rather than exploratory, a doctor recommends it when there is already a clear, focused question to answer, not as a general first search for an unknown genetic cause. This comes up in a few common situations. A clinical picture may already point strongly toward one specific chromosome region, such as a recognizable pattern of physical features and heart or immune differences associated with a well-known microdeletion syndrome. A family member may already have a confirmed genetic change, and other relatives need to be checked for the same one. A pregnancy may need a same-day or next-day answer about a possible common chromosome condition, faster than other tests can provide. Or a cancer diagnosis may need to be confirmed or refined by looking for a specific genetic change known to drive that particular tumor type and to guide treatment choice.
In every one of these situations, fluorescence in situ hybridization answers one focused question quickly and reliably, using cells that do not need to be grown in the laboratory for days beforehand the way some other chromosome tests do. It is best understood as a precise, confirmatory tool aimed at a known target, working alongside broader tests such as chromosomal microarray and genome sequencing rather than replacing them, as later sections explain in more detail.
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