In addition, not like the CD200+ cells in the PHO without synovium hyperplasia, which was mainly identified in the lining layer and to a lesser extent in the sublining layer, the CD200+ cells in the PHO patient with hyperplastic synovium extended to the vascular endothelial regions, whereas CD200R1+ cells were hardly detected (Fig. 4). This evidence concerns the gene CD200 and primary hypertrophic osteoarthropathy.