CTLA4 and breast cancer: Despite somewhat promising results of therapeutic immune checkpoint inhibitors, selection of appropriate agents likely depends on the type of treated cancer, as suggested in recent studies, where tumor cells of breast cancer 4T1 cell line demonstrated significantly lower levels of the CTLA-4 ligand (B7-1) compared to colon carcinoma CT26 cell line, despite similar lower level expression of PD-L1 [22].