EGFR and small cell lung carcinoma: Other mechanisms that confer resistance include amplification of EGFR or activation of other receptor tyrosine kinases, such as ERBB2 and MET, mutation of EGFR downstream signaling effectors, and alteration in cell differentiation status, such as small cell lung cancer transformation, epithelial-to-mesenchymal transition, or acquisition of stem cell–like state (15).