ERBB2 and breast cancer: Nevertheless, under the conditions of pathological overexpression of ErbB2, as it occurs in >20% of invasive breast cancer and, to a lesser extent, in gastric or ovarian cancers2, ErbB2 drives tumorigenesis by spontaneously forming signalling-competent homodimers, ligand-independent heterodimers with ErbB3 as well as larger receptor clusters3, 4, 5, 6.