Delving into the diversity of silencing pathways

Submitted 20 March 2007; accepted 22 May 2007; published online 29 June 2007 developments include the elucidation of steps in the biogenesis of repeat-associated small interfering RNAs (rasiRNAs), new ideas about how siRNAs silence gene expression, how miRNAs repress translation, and the importance of siRNAs as tools in biological research and therapeutics. The discovery of siRNAs, which silence gene expression at the transcriptional and post-transcriptional level in a sequencedependent manner, has revolutionized the biological sciences. Since their discovery, it has become apparent that various classes of siRNA exist, which differ in their origin and mode of biogenesis. They all associate with proteins of the Argonaute family, which mediate their functions. Multicellular organisms encode several Argonaute paralogues, which fall into two main clades: the Argonaute and the PIWI-like proteins (Carmell et al, 2002). The elucidation of steps in the biogenetic pathway of small regulatory RNAs that associate with the PIWI-like proteins was a highlight of this meeting.