ISH techniques are used to identify specific nucleic acid target sequences (DNA or RNA) in a tissue sample. ISH can also identify genetic anomalies that provide diagnostic and prognostic information. In situ methods provide a high information content. There are two main types:
- Fluorescent (FISH)
- Chromogenic (CISH)
CISH and silver enhanced in situ
hybridization (SISH) studies for HER2 determine the presence or absence of gene amplification. In most tumors, the CISH provides the same result as FISH testing. In some unusual cases, different results are obtained by the alternate counting methods, usually due to a variation in the number of chromosome enumerating probe (CEP) 17 signals.
The fluorescent application, FISH, can be used to detect and localize the presence or absence of specific DNA sequences as in CISH methods, but uses a fluorescent chromogen rather than a silver precipitate. FISH is a more established method and is considered the "gold standard." FISH can be used to form a diagnosis, detect diseased cells, evaluate prognosis, or evaluate remission of a disease (cancer). However, both CISH and FISH give quantitative information about the HER2 target.