TRIM21 is a member of the tripartite motif (TRIM) family. The TRIM motif includes three zinc-binding domains, a RING, a B-box type 1 and a B-box type 2, and a coiled-coil region. The 52 kDa Ro protein is part of the RoSSA ribonucleoprotein, which includes a single polypeptide and one of four small RNA molecules. The RoSSA particle localizes to both the cytoplasm and the nucleus. Ro/SSA interacts with autoantigens in patients with Sjogren syndrome and systemic lupus erythematosus. Ribonucleoprotein particle is composed of a single polypeptide and one of four small RNA molecules. The RoSSA is present in all mammalian cells studied but has no known function. At least 2 isoforms are present in nucleated and red blood cells, and tissue specific differences in Ro/SSA proteins were identified Synonyms: 52 kDa Ro protein, Sjoegren syndrome type A antigen, SS-A, Ro(SS-A), 52 kDa ribonucleoprotein autoantigen Ro/SS-A, Tripartite motif-containing protein 21, RING finger protein 81, TRIM21, RNF81, RO52, SSA1, SSA, RO-52