MicroRNAs: a developing story.

Considering the intense genetic efforts applied to understanding development, it is surprising that a relatively large class of regulatory genes has newly surfaced. The first microRNA gene and its developmental role were described more than ten years ago, but only recently have we fully appreciated the broad and abundant presence of such genes. MicroRNAs are approximately 22 nucleotide RNAs that use antisense complementarity to inhibit expression of specific mRNAs. Recent studies of restricted expression patterns and functional roles have implicated specific microRNAs in complex genetic pathways regulating embryogenesis, hematopoiesis, neuronal differentiation and Hox-mediated development.