Drosha is a member of the ribonuclease (RNase) III family. Like other RNase III proteins, Drosha is a double-stranded RNA (dsRNA)-specific endonuclease that introduces staggered cuts on each strand of the RNA helix (1 ,2 ). RNase III proteins are classified based on domain organization. Drosha and its homologs belong to class II, in which each member contains tandem RNase III catalytic motifs and one C-terminal dsRNA-binding domain (dsRBD) (3 ,4 ). Drosha also possesses an extended N-terminal domain whose function is currently unknown. Class I proteins are simpler and possess only one RNase III catalytic motif and a dsRNA-binding domain (dsRBD). The Dicer homologs of class III proteins contain a putative helicase domain, a PAZ domain, and a DUF283 domain apart from tandem nuclease domains and a dsRBD. The human genome encodes only two RNase III proteins: Drosha and Dicer. Whereas Dicer homologs are found in a broad range of eukaryotic organisms, Drosha homologs are present only in animals but not in yeast or plants.