As previously reported by Scaltriti et al. for lapatinib in breast cancer cells [30] and by our group for erlotinib in EGFR wild-type NSCLC cells [14], we demonstrated, in this study, that also osimertinib increased the cell surface expression of HER-2 in EGFR-mutated PC9 in PC9-T790M and in H1975 cell lines. This evidence concerns the gene ERBB2 and breast cancer.