A novel class of endogenous shRNAs in human cells

Until now it is still not clear how many types and amounts of small RNAs (sRNAs) exist in humans. Here we report the identification of 1258 distinct sRNAs derived from intronic regions of protein-coding genes in human with a new approach. These endogenous short hairpin RNAs (shRNAs) appear to be similar to exogenous shRNAs in structure, have a broad distribution in the stem length, and function as microRNAs (miRNAs), small interfering RNAs (siRNA) and/or piwi-interacting RNAs (piRNAs). Except for a few shRNAs, the majority of shRNAs are not phylogenetically conserved. They are differentially expressed in different cells and at diverse developmental stages. Overall, their expression levels are lower than miRNAs', but can be detected by quantitative real-time PCR and microarrays, implying that like other known sRNAs, this type of shRNAs should have important functions in modulating gene expression, and that they may exist in other genomic regions and many species.