For example, breast cancer cells produce chemokines [such as chemokine (C–C motif) ligand (CCL)5 and monocyte chemoattractant protein-1 (MCP-1)] and recruit monocytic cells that then differentiate into protumorigenic macrophages in the presence of interleukin (IL)-4 secreted by type 2 T helper cells to enhance their growth [13]. The gene discussed is CCL2; the disease is breast carcinoma.