TRIM21 and systemic sclerosis: Although the most common SSc-specific antibodies are directed against DNA topoisomerase I (ATA) and the centromeric proteins CENP-A, B, and C (ACA), other autoantibodies such as RNA polymerase III, U3 RNP complex, Th/To, PM/Scl, U1 RNP, Ku, RuvBL1/2, Ro52, NOR90, and ANP32A are also found in patients with SSc [11].