Following the discovery of miRNAs, Drosophila mutants of uncertain provenance that mapped in gain-of-function screens to noncoding regions were re-analysed, and one that regulates growth [136], termed bantam, was identified to encode a miRNA [137]. The miRNAs let-7 [135] and lsy-6 [138] were also identified genetically in C. elegans, not in other organisms, despite the former being not only highly conserved in sequence and expression pattern throughout metazoan evolution [139], but also fundamental to normal and abnormal developmental processes [140]–[142], as are many other miRNAs [143]. On the other hand, lsy-6 is expressed in only a few cells [144], and has only rarely turned up in deep sequencing libraries. Subsequently most known miRNAs, of which there are hundreds in mammals, and later piRNAs, have been identified by biochemical not genetic means. In view of the lsy-6 example and the clearly incomplete sampling of the small RNA transcriptome, even using deep sequencing [70], there are likely to be many more.
