CXCR4 and melanoma: A number of chemokines and chemokine receptors are involved in Treg-mediated homeostasis in melanoma, including chemokine (C-C motif) ligand (CCL)27-CCR10, CXCL12-CXCR4, CCL20-CCR6, CCL19/CCL21-CCR7, CCL9/10/11-CXCR3, CCL17/22-CCR4 and S1P-S1PR1 (Fig. 1) [39, 40, 66–69].