To investigate the potential contribution of LMCs to RA pathogenesis, we recently used the lineage tracing approach and reported that LMCs covering popliteal lymphatic vessels, the joint-draining lymphatic vessels, are Pax7−/MyoD−/Prrx1+/NG2−, which differ from skeletal and vascular muscle cells (Kenney et al. Lineage tracing reveals evidence of a popliteal lymphatic muscle progenitor cell that is distinct from skeletal and vascular muscle progenitors. The gene discussed is PRRX1; the disease is rheumatoid arthritis.