 In non-small celled lung cancer NSCLC 10-15 of the patients harbor a mutation in the tumors epidermal growth factor receptor EGFR M. This receptor is the target for treatment with erlotinib. Identification of EGFR M is done on a biopsy, which can be difficult to retrieve. 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. A pilot study demonstrated that the quantity of EGFR M in plasma correlates to the response to treatment and might be used to predict disease progression. 