■ #HER2 protein overexpression is virtually always caused by amplification of the region of chromosome 17q that contains the HER2 gene.
■ #The increase in HER2 gene copy number is detected by fluorescence in situ hybridization (FISH) using a HER2-specific
probe (red signal), which is typically cohybridized to tumor cell nuclei with a second probe specific for the centromeric region of chromosome 17 (green
signal), allowing the chromosome 17 copy number to be determined.
■ #Alternatively, HER2 protein overexpression in tumor cells can be detected by immuno-
histochemical staining with antibodies specific for HER2.
