CTLA4 and breast cancer: Although CTLA-4 expression is mainly expressed on immune cells, but can also be expressed on tumor cells, such as those found in acute myeloid leukemia, chronic myeloid leukemia, B cell leukemia, melanoma and breast cancers can also express CTLA-4, as well, and the expression of CTLA-4 on tumor cells can trigger apoptosis upon ligand interaction [73,74,104,105,106].