The monitoring of plasmatic tumor DNA in endocrine-resistant breast cancer patients showed that 8.4% of these patients developed novel HER2 mutations; interestingly, the treatment of one HER2-mutant patient with the HER2 irreversible inhibitor Neritinib resulted in a remarkable clinical response, with the disappearance of two tumor clones bearing HER2 mutations, but the persistence of other passenger subclones [190]. The gene discussed is ERBB2; the disease is neoplasm.