BTK and breast cancer: Similarly, numerous studies have shown that BTK inhibition causes substantial cytotoxicity to HER2+ breast cancer cells, inhibits their proliferation and clonogenicity, and diminishes their resistance to chemotherapy both in vitro and in vivo (Eifert et al., 2013; Chen et al., 2016; Wang et al., 2016; Metzler et al., 2020; Wen et al., 2020).