Background: Endogenous small non-coding RNAs have been found to play pivotal roles in regulating gene expression in eukaryotes.While huge interests are put into the function and molecular mechanism of microRNAs in the development and disease of various organisms via the repression of mRNA of protein coding gene, new discoveries indicate that they may trigger the generation of another type of small RNAs, the trans-acting siRNAs (ta-siRNAs).This implies a new mode of RNA function, the regulation between small RNAs which gives rise to an even more complicate and elaborate pattern of RNA regulation mechanism for gene expression.