BTK and breast cancer: Similar to that observed in CLL patients, ibrutinib treatment results in a significant reduction of MDSCs in the spleen and tumor in WT mice transplanted with mammary tumors, melanomas or neuroblastomas, but not in transplanted XID mice harboring a BTK mutation, suggesting a BTK-dependent mechanism of action for ibrutinib on MDSCs (Stiff et al., 2016; Varikuti et al., 2020; Ishfaq et al., 2021).