Table 1. Small RNAs Involved in Gene Silencing.

class	size (nt)	function	origin	species
microRNAs	21–25	translation repression and mRNA destabilization	miR-coding genes, introns (mirtrons)	C. elegans, D. melanogaster, A. thaliana, O. sativa, mammals
endo-siRNAs	21–25	transposon silencing and mRNA degradation	transposons, pseudogenes	D. melanogaster, C. elegans, mammals
siRNAs	21–25	mRNA degradation and transposon silencing	intergenic regions, exons and introns	C. elegans, D. melanogaster, S. pombe, A. thaliana, O. sativa
piRNAs	24–31a	transposon silencing, spermatogenesis	transposons and other repeat sequences	C. elegans, D. melanogaster, Danio rerio, mammals

^a piRNAs in C. elegans are ∼21 nt in length.