In a breast cancer model (PyMT/Tgfbr2KO mammary carcinoma), a mutated TGF-β receptor causes recruitment of CD11b+ Gr-1+ MDSC at the tumor invasion front via the CXCL5–CXCR2 and CXCL12–CXCR4 axes, and contributes to tumor invasion and lung metastasis through metalloproteinase (MMP) secretion [73]. Here, CXCR4 is linked to breast cancer.