Mobile silencing RNAs and their activity. Mature miRNAs (black) are probably mobile because they appear in recipient neighboring cells, phloem and distant tissue. Their physiologically active mobile form is not conclusively known, however, and might include a mobile precursor RNA. siRNAs (red) move from cell to cell and over long distances to direct RNA cleavage and DNA methylation. However, many aspects of this process remain unknown as indicated by question marks. There is evidence that miRNAs and siRNAs can move from the shoot to the root; however, very little is known about the reciprocal movement and activity of mobile sRNAs.