Gene expression is mediated by a wide variety of RNA molecules. Messenger RNAs (mRNAs) are protein-coding RNAs, which are involved in translation. There are RNA species produced by eukaryotes, classified as non-coding RNAs, which are not involved in translation but play a major role in regulation of gene expression at the transcriptional, post-transcriptional and translational levels. The small non-coding RNA molecules along with the Argonaute family of proteins have been identified as key players in various forms of sequence specific gene silencing, including RNA interference (RNAi), translational repression and heterochromatin formation. Argonaute proteins are composed of the N-terminal, PAZ (Piwi-Argonaute-Zwille), middle and the C-terminal PIWI (P-element-induced wimpy testis) domains (Tolia et al., 2007).
    Five genes have been found to encode distinct members of the Argonaute family of proteins in Drosophila, namely AGO1, AGO2, Aubergine (Aub), Piwi and AGO3 (Gunawardane et al., 2007), of which AGO1 and AGO2 belong to the Argonaute (AGO) subfamily. The remaining three proteins Aub, Piwi and AGO3 occur abundantly in the germline cells and are grouped under the PIWI subfamily of proteins. They play a vital role in the formation of germline cells and also exhibit target RNA cleavage by a process called Slicing, thereby silencing retrotransposons and other repeat elements in the germline.
    These proteins act as molecular scaffolds that present the small, guide RNA molecules of RNA-silencing to their complementary targets, by forming a ribonucleoprotein complex. Several types of small RNAs have been classified based on their biogenesis and mode of action. The various classes of small RNAs, their source, length and functions have been listed in Table 1.