This diagram represents a positive fluorescence in situ hybridization (FISH) assay for for C-erb B-2 (HER-2/neu) in a breast carcinoma. Note that within the cells the probe which marks in red for amplification of the HER-2 gene is in much greater quantity than the green probe marking for a normal component of a pair of chromosomes. HER-2 oncogene overexpression is typically the result of gene amplification (more gene copies) and is detected by the FISH assay, but in a few cases may be due to transcription activation.

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