Abstract
Micro RNAs are a large family of noncoding RNAs of 21–22 nucleotides whose functions are generally unknown. Here a large subset of Drosophila micro RNAs is shown to be perfectly complementary to several classes of sequence motif previously demonstrated to mediate negative post-transcriptional regulation. These findings suggest a more general role for micro RNAs in gene regulation through the formation of RNA duplexes.