ERBB2 and cancer: The receptor is encoded by the proto-oncogene HER2/ERBB2 located on Chromosome 17 that can undergo massive copy number gains of as much as 25–50 gene copies, resulting in the expression of millions of HER2-RTKs on cancer cell surfaces.29HER2/ERBB2 gene amplification is recognized as an oncogenic driver in breast, ovarian, gastric, gastroesophageal, endometrium, bladder, lung, colon, and head and neck cancer cells.