Of note, not like the CD200+ cells in the PHO patient without synovium hyperplasia, which was mainly identified in the lining layer and to a lesser extent in the sublining layer, the CD200+ cells in this PHO patient with hyperplastic synovium extended to the vascular endothelial regions, whereas CD200R1+ cells were hardly detected. Here, CD200R1 is linked to primary hypertrophic osteoarthropathy.