They are recruited by chemotactic signals such as colony-stimulating factor-1 (CSF1) and chemokine (C-C motif) ligand-2 (CCL2) released by the tumor and/or non-tumor cells in the TME (Roca et al., 2009; Lin et al., 2013; Leone and Powell, 2020). The gene discussed is CCL2; the disease is neoplasm.