Table 1.

Cellular small RNAs involved in gene silencing

Class	Size (nucleotides)	Functions	Mechanisms	Origin	Organisms found in
siRNAs	21–25	Regulating gene expression, providing antiviral response, restricting transposons	RNA degradation, transposon restriction	Intergenic regions, exons, introns	Caenorhabditis elegans, Drosophila melanogaster, Schizosaccharomyces pombe, Arabidopsis thaliana, Oryza sativa (rice)
endo-siRNAs	21–25	Restricting transposons, regulating mRNAs and heterochromatin	RNA degradation	Transposable elements, pseudogenes	D. melanogaster, mammals
miRNAs	21–25	Regulating gene expression at the post-transcriptional level	Blocking translation, RNA degradation	Intergenic regions, introns	C. elegans, D. melanogaster, S. pombe, A. thaliana, O. sativa, mammals
piRNAs	24–31*	Regulating germline development and integrity, silencing selfish DNA	Unknown	Defective transposon sequences and other repeats	C. elegans, D. melanogaster, Danio rerio, mammals
ra-siRNAs	23–28	Remodelling chromatin, transcriptional gene silencing	Unknown	Repeated sequence elements (subset of piRNAs)	C. elegans, D. melanogaster, S. pombe, Trypanosoma brucei, D. rerio, A. thaliana
ta-siRNAs	21–22	Trans-acting cleavage of endogenous mRNAs	RNA degradation	Non-coding endogenous transcripts	D. melanogaster, S. pombe, A. thaliana, O. sativa
natRNAs	21–22	Regulating gene expression at the post-transcriptional level	RNA degradation	Convergent partly overlapping transcripts	A. thaliana
scnRNAs	26–30	Regulating chromatin structure	DNA elimination	Meiotic micronuclei	Tetrahymena thermophila, Paramecium tetraurelia
tncRNAs	22	Unknown	Unknown	Non-coding regions	C. elegans

^*C. elegans piRNAs are 21 nucleotides. endo-siRNAs, endogenous siRNAs; miRNAs, microRNAs; natRNAs, natural antisense transcript siRNAs; piRNAs, PIWI-interacting RNAs; ra-siRNAs, repeat-associated siRNAs; scnRNAs, scan RNAs; siRNAs, short interfering RNAs; ta-siRNAs, trans-acting siRNAs; tncRNAs, tiny non-coding RNAs.