They are directed to the tumor site by chemotactic cues produced by cancer cells or stromal cells, including vascular endothelial growth factor (VEGF), colony stimulating factor-1 (CSF-1), placental growth factor, and chemokines, such as the chemokine (C-C motif) ligand (CCL) 2/monocyte chemotactic protein-1, CCL3/macrophage inflammatory protein-1alpha, CCL4/macrophage inflammatory protein-1beta, and the CCL5/regulated on activation, normal T-cell expressed and secreted (RANTES) (28, 29). Here, CCL4 is linked to cancer.