Abstract: MicroRNAs (miRNA),one of the small moleculay RNA family, are single strand endogenous noncoding 21 to 23 nucleotide RNA molecules that keep high conservation in the process of evolution. Although they are tiny,In eukaryotes,microRNAs played important roles in the course of development and gene expression regulation, typically by forming perfect or imperfect duplexes with target messenger RNAs. In 2004, Epstein-Barr virus, was the first virus shown to encode and express miRNAs. Recently, miRNAs have also been cloned from Kaposi's sarcoma-associated herpesvirus, mouse γherpesvirus68 and human cytomegalovirus. In addition, the SV40 miRNA have been shown to negatively regulate large T-antigen （TAg） expression. This review will summarize recent findings on the prediction, expression and functions of miRNA and virally encoded miRNAs.

Key words: miRNA; CTL; siRNA; virology