Table 1.
Noncoding RNA classifications
| Class | Size in bases | Location | Functional role | Example |
|---|---|---|---|---|
| Small106–108 | miRNAs (~19–24) | Intracellular, extracellular | Post-transcriptional regulation of gene expression by degradation or translation repression of target mRNAs | miRNA-21 |
| siRNA (~21–23) | Exogenous | Silencing gene expression by degradation of target mRNAs | TTR-siRNA109 | |
| piRNAs (~26–31) | Repression of reproductive cell-specific gene expression | piRNAs targeting LINE1 elements | ||
| Intermediate107,110,111 | snoRNAs (~20–200) | Intra-nucleolus | Modification and processing of rRNA precursors | SNORD |
| snRNAs | Intra-nuclear | Nuclear maturation of primary transcripts of mRNAs, RNA splicing | U snRNA | |
| Long106,112 | >200 | Nucleus, cytoplasm, mitochondria | Maintaining inactive chromatin as scaffold by repressing genes including HOXD genes | HOTAIR (>200) |
| X-chromosome inactivation | XIST (~17 kb) | |||
| Repression of XIST (antisense transcript of XIST) | TSIX (>200) |
Abbreviations: HOTAIR, (HOX) transcript antisense RNA; HOX, a subset of homeotic genes; miRNA, microRNA; mRNA, messenger RNA; piRNA, piwi-interacting RNA; rRNA, ribosomal RNA; siRNA, small interfering RNA; snoRNA, small nucleolar RNA; snRNA, small nuclear RNA; TSIX, XIST antisense RNA; XIST, X-inactive specific transcript.