MX1 and neoplasm: The association of F4/80+ macrophages with B16F1 tumor cells, on the other hand, was negligible in lungs from pFN-competent C57BL/6-Fn(fl/fl)Mx-Cre− as well as pFN-deficient C57BL/6-Fn(fl/fl)Mx-Cre+ mice harvested 1 h after tumor cell injection but increased considerably after 16 h (Figure 2C,D).