Although DPP-IV activity levels are decreased both in serum and synovial fluid of RA patients [11,4,6–10] and this decrease is known to be associated to disease activity [8,10], little is known about the events leading to this decrease, apart from the fact that RA patients display higher percentages of CD4+CD26+ T cells and CD26 cell surface density [15,18], an opposite situation when compared to SLE patients [6]. The gene discussed is DPP4; the disease is rheumatoid arthritis.