The expression profile of microRNAs in mouse embryos

MicroRNAs (miRNAs), which are non-coding RNAs 18–25 nt in length, regulate a variety of biological processes, including vertebrate development. To identify new species of miRNA and to simultaneously obtain a comprehensive quantitative profile of small RNA expression in mouse embryos, we used the massively parallel signature sequencing tech-nology that potentially identifies virtually all of the small RNAs in a sample. This approach allowed us to detect a total of 390 miRNAs, including 195 known miRNAs covering 80 % of previously regis-tered mouse miRNAs as well as 195 new miRNAs, which are so far unknown in mouse. Some of these miRNAs showed temporal expression profiles during prenatal development (E9.5, E10.5 and E11.5). Several miRNAs were positioned in poly-cistron clusters, including one particular large tran-scription unit consisting of 16 known and 23 new miRNAs. Our results indicate existence of a sig-nificant number of new miRNAs expressed at specific stages of mammalian embryonic development and which were not detected by earlier methods