Associations between co-expression of PEA3 and HER2 and reduced disease-free survival and between upregulation of PEA3 protein expression, in primary cultures, and both axillary lymph node positivity and HER2 overexpression are suggestive of a positive role for PEA3 in HER2-mediated breast tumour progression. This evidence concerns the gene ETV4 and breast neoplasm.