Approaches to microRNA discovery.

MicroRNAs (miRNAs) are noncoding RNAs that can regulate gene expression. Several hundred genes encoding miRNAs have been experimentally identified in animals, and many more are predicted by computational methods. How can new miRNAs be discovered and distinguished from other types of small RNA? Here we summarize current methods for identifying and validating miRNAs and discuss criteria used to define an miRNA.