To detect resistance gene from serially collected plasma DNA in non-small cell lung cancer harbouring EGFR activating mutation who are being treated with EGFR TKIs by using castPCR method.

Inclusion Criteria:

• 1. Histologically confirmed NSCLC 2. Aged of or older than 20 years 3. ECOG performance status 0-2 4. Adequate hematological, renal, hepatic function 5. Patients with tumors harboring EGFR mutation (del 19 or L858R mutation) 6. Patient who are about to be treated with EGFR TKI (gefitinib, erlotinib or other EGFR TKI) 7. At least more than one measurable disease 8. Informed consent

Exclusion Criteria:

• 1. Active infection 2. Active bleeding