Using the PyMT breast cancer mouse model, which develops palpable breast cancers that metastasize to the lung, Qian et al. reported that IMs, which are characterized by Gr-1/Ly6C+, CD11b/CD115+ markers, play a key role in the lung metastasis of breast cancer in a C–C motif chemokine ligand 2 (CCL2)-dependent manner [81]. The gene discussed is CCL2; the disease is breast cancer.