The specific factors produced by 67NR and 4T1 mammary tumor lines that are responsible for driving the distinct phenotypic changes in tumor-bearing SHIP−/− mice are the topic of ongoing research, although 4T1 tumor cells are known to be an abundant source of G-CSF [26, 27] and mice with 4T1, but not 67NR, tumors exhibit increased serum levels of G-CSF, but not GM-CSF or M-CSF (Bosiljcic, Hamilton, and Bennewith unpublished data). Here, CSF1 is linked to breast cancer.