Although we found no evidence of C3aR or C5aR1 expression by EMT6 or 4T1 mammary tumor cells, expression of these receptors has been reported for a range of human and mouse tumor cells, including melanoma [27], lung [53] and ovarian [31] cancers—all of which are inhibited by C3aR or C5aR1 receptor antagonism. This evidence concerns the gene C3AR1 and breast cancer.