MAP3K knockdown by small interfering RNA (siRNA) in rheumatoid arthritis fibroblast-like synoviocytes (FLSs). Cultured FLSs were transfected with 1 or 5 μg of MEKK1, MEKK2, MEKK3, TAK1 or scrambled negative control (sc) siRNA as described in Materials and methods. FLSs were then incubated for 3, 5 and 7 days and western blot analysis was performed. siRNA specifically inhibited respective kinase expression by >75%. Optimal knockdown of MEKK1 (5 μg siRNA), MEKK2 (1 μg siRNA) and MEKK3 (1 μg siRNA) was observed on day 3 and on day 5 for TAK1 (1 μg siRNA).