The human epidermal growth factor receptor 2, HER2 or ERBB2 (hereafter called HER2) is a well-characterised oncogene and amplification and/or protein overexpression of HER2 is common in human cancers of the breast, ovary, lung, stomach, colon, pancreas, and endometrium (for a review see ref. 1). The gene discussed is ERBB2; the disease is cancer.