Recent studies have shown that si RNA (short interfering RNA exogenous or endogenous 19 to 25 nt (Nucleotides) has a role in regulating the gene expression. How the 21 nucleotide sequences of RNA have a regulatory function. It has been reported to have ds RNA form which could split the mRNA and thus influence the gene expression.For detials consult the Ref: P.DUNOYER, O.VOINNET, Trends Plant Science. 14:643 (2009)