We measured the expression of PIWIL1-4 mRNA in cultured SF and could detect similar high expression of PIWIL2 and PIWIL4 mRNA in cells from both RA and OA patients (mean PIWIL2 dCt±SEM in RA 2.2±0.8 and in OA 1.3±0.6; mean PIWIL4 dCt±SEM in RA 1.3±0.3 and in OA 1.2±0.4).(Fig 1A2). Here, PIWIL4 is linked to rheumatoid arthritis.