Although the role of increased Src activity in the resistance of ERBB2-amplified breast cancer and GE cancer to ERBB2 inhibition has been documented, we reported here, for the first time, the activation of a spontaneous Src mutation after prolonged exposure to HER2 inhibitor could induce lapatinib resistance in ERBB2-amplified GE adenocarcinoma and present the first evidence of acquired mutation of Src as an etiology of resistance. This evidence concerns the gene SRC and breast carcinoma.