CXCR4 and neoplasm: As CXCR4 is thought to promote migration to the lungs, liver, and bone marrow, other chemokines are associated with metastasis to specific organs: functionally active CCR9 facilitates metastasis of mouse tumour cells to the small bowel [78], and CCR4 appears to play an important role in metastasis to the brain [68], the site at which CCL22, one of its ligands, is expressed.