CXCR4 and pulmonary fibrosis: The release of fibrocytes from the BM into the bloodstream is mediated by the interaction of the chemokine receptor CXCR4 — which is expressed by most fibrocytes (4, 7, 14–16) — and its ligand, CXCL12, and blocking this interaction in murine models of pulmonary fibrosis results in reduced traffic of fibrocytes to the lungs and attenuated fibrosis (4, 15, 17–19).