Deciphering Afatinib Response and Resistance With INtratumour Heterogeneity

To assess if targeting activating EGFR and HER2 mutations in Non-Small Cell Lung Cancer (NSCLC) is more effective when these mutations are truncal dominant mutations (≥50%), as opposed to non-dominant (≥5 to \<50%) or low frequency mutations (\<5%). This trial will be available to patients registered to the TRACERx study (NCT01888601), or non-TRACERx patients who have two archival tissue/DNA samples who are willing to have a biopsy of their relapsed disease.