Locus-specific probes target a specific gene sequence of interest. These probes can be used to determine whether a gene is amplified, deleted, or present in a normal copy number.
Amplification is usually determined by comparison to the number of centromeres in the same cell. The ratio of gene to centromere number is used frequently in reporting whether a tumor is amplified or deleted for a particular gene.
The optimal way to determine if a gene is truly amplified is to have a probe set with both the locus-specific probe and the centromere probe for the same chromosome. This procedure is used in many types of FISH testing, such as:
- Human epidermal growth factor receptor 2 (HER2/neu)
- Epidermal growth factor receptor (EGFR)
- Myelocytomatosis cellular oncogene (C-MYC)
Deletion involves the loss of a segment of a chomosome.
In the image, the chromosome on the left contains 1 copy of a gene (locus specific probe) that is tagged with a red fluorescent dye and would be read as 1 red signal. The chromosome on the right contains 3 copies of a gene (locus specific probe) that is tagged with a red fluorescent dye and would be read as 3 red signals.