PECAM-1^−/− MKs demonstrate defective polarization of the chemokine receptor CXCR4. CXCR4 immunostaining of migrating PECAM-1^+/+ MKs and PECAM-1^−/− MKs toward the SDF1α source (as indicated by the arrow) demonstrates defective polarization. Scale bar = 20 μm. Image acquisition was performed as described for Figure 2C.