TNF and Arthritis: To explore the possible mechanisms underlying the reduced arthritis observed in PARP-1 knockout mice compared to control mice, we studied, by quantitative real-time PCR, mRNA levels of IL-1β, IL-6, TNF-α, MCP-1, small inducible cytokine A5 (Cc15; RANTES), inducible nitric oxide synthase (iNOS) and cyclooxygenase (COX)-2 in arthritic joints at day seven after Arthrogen injection.