Another biopsy was performed on a progressing liver lesion and the tissue was sent for NGS using Foundation One CDx testing. This biopsy revealed the emergence of the EGFRT790M mutation (allelic frequency of 59%). Interestingly, this was accompanied by an increase in the allelic frequency of the L718V mutation (80%), and a decrease in the frequency of the L718Q mutation (3%). This report suggests that while L718Q/V mutant tumors could benefit from transient responses to afatinib treatment, they can acquire the T790M mutation and eventually become resistant to all available EGFR TKIs.