CCL21 and neoplasm: CCR4 is highly expressed on Treg relative to conventional T cells and facilitates Treg migration toward tumours expressing the CCR4 ligands CCL17/CCL21 [49–51], while other studies have shown that Tregs expressing VEGF-A (CXCR4) are preferentially attracted to tumours expressing the angiogenic promoting chemokine CXCL12 [52, 53].