Given the fact that ibrutinib may have relevant HER2 targeting activity, Chen et al. [2] sought to address several critical questions regarding its potential clinical use in HER2-overexpressing breast cancer through growth inhibition experiments, apoptosis assays, xenograft mouse models, pharmacokinetics/pharmacodynamics, immunohistochemical analysis, Western blot analysis, cell cycle assay, BTK occupancy assays, and kinase assays. This evidence concerns the gene ERBB2 and breast cancer.