CD200R1 and primary hypertrophic osteoarthropathy: Histological examination of the synovium obtained from his right knee revealed endothelial hyperplasia and vascular thickening without inflammation that was in association with aberrant expression of CD200/CD200R1, which highlighted the importance of aberrant CD200/CD200R1 in the regulation of the endothelial activation that contributed to the development of synovium hyperplasia in this PHO patient.