ERBB2 and breast cancer: Reduction-sensitive dimerization is not the only mechanism by which ERBB2 extracellular domain mutations constitutively activate enzymatic activity; ERBB2 S310F and S310Y mutations, found in 1–2% of lung cancers and breast cancers, behave more similarly to the ERBB2 kinase domain mutants in that they cause elevated C-terminal tail phosphorylation without evidence of covalent dimerization.