Although the functional role of non-coding poly(dA:dT) is well established, the mechanism underlying this function remains largely unknown. In addition to the theories about the intrinsic properties of poly(dA:dT), it is believed that these poly(dA:dT) tracts may serve as cisregulatory elements or binding sites for trans-acting factors. Protein complexes that form with a certain repeat sequence may regulate specific biological functions. However, no trans-acting poly(dA:dT) binding proteins have been reported to date. Currently, it is well accepted that small RNAs are key players in target recognition. In addition, small RNAs can play a regulatory role in controlling gene expression (18). The discovery of RNA interference (RNAi), for which the 2006 Nobel Prize in Physiology was awarded, suggests that small RNAs play important roles in epigenetics (19,20). RNAi is characterized by the binding of a small interfering RNA to a messenger RNA (mRNA), which targets that mRNA for degradation. As a result, the corresponding gene is downregulated. During the first step of the RNAi pathway, double-stranded RNAs or pre-microRNAs are cleaved by the Dicer protein into small doublestranded RNA fragments (20–25bp). Second, a singlestranded RNA is selected by Argonaute proteins and