The present invention is directed to a method for determining the responsiveness of cancer to an epidermal growth factor receptor (EGFR) treatment. In a preferred embodiment, the presence of at least one variance in the kinase domain of the erbB1 gene confers sensitivity to the tyrosine kinase inhibitor gefitinib. Thus, a diagnostic assay for these mutations will allow for the administration of gefitinib, erlotinib and other tyrosine kinase inhibitors to those patients most likely to respond to the drug.