Introduction

MicroRNAs are a novel class of small endogenous non-coding RNAs, in their mature form typically of length 21–23 nt. They suppress protein production by binding to the mRNA of their target genes. The apparent fundamental role of microRNAs in nematode development, together with the outstanding degree of conservation of let-7, triggered large-scale searches for microRNAs in various organisms. In the order of at least 100–200 microRNAs per species have already been identified experimentally in C. elegans, D. melanogaster, D. rerio, M. musculus and H. sapiens, many of them being conserved over large evolutionary distances. Recent computational searches for microRNA genes in humans indicate that the true number of different microRNAs per species in mammals could exceed 500, including a substantial fraction of species or lineage specific microRNAs (Berezikov et al., 2005). After the discovery of hundreds of microRNAs in a variety of plants and animals this class of small non-coding RNAs turns out to be an important player in post-transcriptional gene regulation. Mature microRNAs display complex expression patterns during development and in adult tissues. Some were specifically expressed at high numbers, others appear to be expressed in almost all tissues (Barad et al., 2004; Baskerville and Bartel, 2005), suggesting substantial microRNA regulation in various physiological processes. The discovery and biogenesis of microRNAs is presented in other chapters of this book.