 Identification of EGFR M is done on a tumor biopsy, which can be difficult to retrieve. Furthermore a biopsy only provides information about a certain part of the one tumor site, that is tested. The investigators know that tumors especially in NSCLC is a very heterogenous group. A blood test provides us with e more overall information about the tumor mass of each specific patient. However tumor DNA is also present in blood of cancer patients in the form of circulating free DNA. A new blood based test identifies EGFR M in plasma, which makes it possible to monitor the level of EGFR M in the patients blood during treatment. This enables both a closer monitoring of the treatment with erlotinib and a closer study of the resistance mechanisms that almost inevitably develop during treatment. 