TABLE 1.

Comparison of different small RNAs.

Name	Length (nt)	Strand	Occurrence	Complementarity with target mRNA	Function	Evolutionary conservation
miRNA	17–25	Single-stranded	Plants and animals	Partially complementary, so a single miRNA can target hundreds of mRNA types	Regulation of gene translation	Almost conservative in the species
siRNA	21–23	Double-stranded	Plants and lower animals, but mammals do not	Fully complementary, targeting only one mRNA type	Protection against virus intrusion	Hardly conservative in the species
piRNA	24–31	Single-stranded	Metazoans, mostly germline	—	Genome stabilization	Not very conserved

Nt, nucleotide; miRNA, microRNA; siRNA, small interfering RNA; piRNA, Piwi-interacting RNA.