Apart from its mRNA expression in knee OA FLS, this study examined IL-34 mRNA expression in knee OA synovium with regard to the severity of synovitis and revealed up-regulation of IL-34 mRNA expression in severe synovitis of knee OA synovium, consistent with IL-6 and IκB mRNA expression analyses demonstrating significant up-regulations of their mRNA expressions in severe synovitis of knee OA synovium. The gene discussed is IL34; the disease is synovitis.