TRIM21 and viral infectious disease: An increasing number of studies have shown that TRIM proteins, such as TRIM5 (19), TRIM21 (20), TRIM23 (21), TRIM29 (22), TRIM31 (23), TRIM32 (24), TRIM56 (25), TRIM65 (26), and TRIM26 (27) positively regulate IFNs in immunological effectors of innate immunity against viral infections; whereas TRIM40 (28) negatively regulates the immune response.