Human epidermal growth factor receptor 2 or HER2 is a cell surface protein that is involved in the growth of normal cells. When the amounts of HER2 become high, it can cause uncontrolled cell growth. Tumors with unusually high levels of HER2, ie, those with an overexpression or amplification of HER2, are said to be HER2 positive. These are treated with a specific type of therapy. A simple test can measure the levels of HER2 in your tumor and determine if you are a candidate for this type of therapy.1,2
 Various testing methods may be used to identify specific genes, proteins, or other factors that are unique to the tumor. These methodologies are called molecular testing and often begin with immunohistochemistry (IHC) or fluorescence in situ hybridization (FISH). Both techniques use biopsy tissue or samples from surgery for testing. Depending on the results of IHC, additional testing with FISH may be necessary, particularly if IHC does not give a clear positive or negative. Next-generation sequencing (NGS) is typically used when there is not enough diagnostic tissue or traditional biopsy is not able to be performed, leaving no suitable samples for IHC/FISH, as well as when multiple biomarkers need to be detected.3-6
The National Comprehensive Cancer Network (NCCN) guidelines recommend molecular testing for HER2 overexpression and/or amplification in inoperable, locally advanced, recurrent, or metastatic GEA and unresectable or metastatic BTCs including gallbladder, intrahepatic CCA, and extrahepatic CCA, due to the significant incidence of HER2 overexpression and/or amplification in these types of cancers. HER2 amplification can be detected by IHC, FISH, or NGS techniques, with NGS being considered upfront in the case of limited availability of diagnostic tissue.4-5
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