Several recent studies have identified CAFs that drive a collagen ECM synthesis program in TAMs that is tightly regulated by TGFβ in breast cancer.[24] Inflammatory monocytes and tissue‐resident macrophages are key regulators of tissue fibrosis, playing major roles in the initiation, maintenance, and resolution of tissue injury.[49] Therefore, we set out to investigate whether ECM, especially collagen, limits antitumor immunity and responsiveness to chemotherapy. This evidence concerns the gene TGFB1 and breast cancer.