In lung cancers that are sensitive to EGFR inhibitors, ErbB3 mediates PI3K activation; thus, ErbB3 redundant activation, induced by MET amplification, enables cancer cells to transmit the same downstream signaling even in the presence of EGFR inhibitors and concomitant inhibition of both EGFR and MET is required to kill these resistant cells. The gene discussed is ERBB3; the disease is lung cancer.