Small temporal RNAs in animal development.

The lin-4/miR-125 and let-7 microRNAs are at the heart of the heterochronic pathway, which controls temporal cell fate determination during Caenorhabditis elegans development. These small temporal RNAs are clustered along with a third microRNA, miR-100, in the genomes of most animals. Their conserved temporal and neural expression profile suggests a general role in cell fate determination during nervous system differentiation. By triggering consecutive differentiation programs, these microRNAs probably help to determine birth-order dependent temporal identity and thereby contribute to neural stem cell multipotency.