An abundant class of tiny RNAs with probable regulatory roles in Caenorhabditis elegans

Two small temporal RNAs (stRNAs), lin-4 and let-7, control developmental ti ming in Caenorhabditis elegans. We find that these two regulatory RNAs are members of a large class of 21- to 24-nucleotide noncoding RNAs, called mic roRNAs (miRNAs). We report on 55 previously unknown miRNAs in C. elegans. T he miRNAs have diverse expression patterns during development: a let-7 para log is temporally coexpressed with let-7; miRNAs encoded in a single genomi c cluster are coexpressed during embryogenesis; and still other miRNAs are expressed constitutively throughout development. Potential orthologs of sev eral of these miRNA genes were identified in Drosophila and human genomes. The abundance of these tiny RNAs, their expression patterns, and their evol utionary conservation imply that, as a class, miRNAs have broad regulatory functions in animals.