Revolution in biotechnology made possible to identify those gene errors, which via their encoded proteins (mostly kinase enzymes) are key players in tumor development, growth and progression, and could be considered as molecular targets in tumor diagnosis and therapy. Activity of EGFR (epidermal growth factor receptor), an outstanding representative of the regulatory cell surface receptors, can be inhibited by drugs proved for clinical use. In the past year many groups observed that those lung adenocarcinoma cells, which contain activating mutation in the tyrosine kinase domain of EGFR show remarkable sensitivity to anti-EGFR compounds. The basis of the effective therapy is the identification of the mutations. The clinical advantage of EGFR is an example from the coming age of tumor chemotherapy, when the presence of molecular targets will guide the therapeutic choice.