The biological axis of the receptor CXCR4 and its ligand CXCL12 also plays an important role in the homing of bone marrow-derived progenitor cells [41], and a direct correlation exists between plasma and lung levels of CXCL12 and circulating and lung fibrocyte numbers in human IPF [42]. Here, CXCL12 is linked to idiopathic pulmonary fibrosis.