Tripartite motif-containing protein 21, also known as E3 ubiquitin-protein ligase TRIM21, is a protein that in humans is encoded by the TRIM21 gene. Alternatively spliced transcript variants for this gene have been described but the full-length nature of only one has been determined. It is expressed in most human tissues. TRIM21 is an intracellular antibody effector in the intracellular antibody-mediated proteolysis pathway. It recognizes Fc domain and binds to immunoglobulin G, immunoglobulin A and immunoglobulin M on antibody marked non-enveloped virions which have infected the cell. Either by autoubiquitination or by ubiquitination of a cofactor, it is then responsible for directing the virions to the proteasome. TRIM21 itself is not degraded in the proteasome unlike both the viral capsid and the bound antibody. TRIM21 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.