EGFR and metastatic neoplasm: Considering that most of the studies have been conducted using tissue obtained from the primary tumor whereas EGFR monoclonal antibodies have been used to treat the metastatic disease, it is possible that the lack of efficacy and/or the emergence of subsequent resistance may be due to genetic diversification of metastatic cells compared to their primary tumor counterparts or to dynamic variations in tumor genotype or phenotype that emerge during treatment.