Table 1.

Various classes of ncRNAs

	Size (bp)	Current number*	Putative functions
Small ncRNAs
tiRNA	17–18	>5,000	Transcriptional initiation
miRNA	18–25	2042	Translational repression; Transcriptional activation
siRNA	19–25	>20,000	mRNA degradation
tasiRNA	20–22	unknown	Gene silencing in plants
tel-sRNA	23–28	unknown	Epigenetic regulation of telomerase
rasiRNA	24–29	>1,000	Transposon silencing
piRNA	26–31	>60,000	Transposon silencing
CRISPR	24–48	Unknown	Prokaryotic immune control
crasiRNA	34–42	Unknown	Heterochromatin recruitment
Medium-size ncRNAs
TSS-aRNA	20–90	>10,000	Transcriptional regulation
PASR	22–200	>10,000	Transcriptional regulation
snoRNA	60–300	>300	Maturation of other ncRNAs
scaRNA	83–330	>26	Guiding spliceosomal RNAs
Long ncRNAs
lncRNA	>200	>10,000	Transcriptional regulation
T-UCR	200–779	481	Antisense inhibition of mRNAs and ncRNAs
CUT	200–800	>900	Chromatin regulation
SUT	200–800	>800	Transposon silencing
TERRA	100–9,000	unknown	Regulation of telomere length
PROMPT	>200	unknown	Promoter control

^*Current number in humans is given as of today. New ncRNAs of all classes are still being discovered. tiRNA, transcription initiation RNA; miRNA, microRNA; siRNA, short interfering RNA; tasiRNA, trans-acting siRNA; tel-sRNA, telomere small RNA; rasiRNA, repeat-associated siRNA; piRNA, piwi-interacting RNA; CRISPR, clustered regularly interspaced short palindromic repeats; crasiRNA, centromere repeat associated short interfering RNA; TSS-aRNA, transcription start site associated RNA; PASR, promoter associated small RNA; snoRNA, small nucleolar RNA; scaRNA, small Cajal body-specific RNA; lncRNA, long noncoding RNA; T-UCR, transcribed ultraconserved region; CUT, cryptic unstable transcript; SUT, stable unannotated transcript; TERRA, telomere-associated ncRNA; PROMPT, promoter associated pervasive transcript.