Non-coding RNA  (非编码RNA)

The term non-coding RNA (ncRNA) is commonly employed for RNA that does not encode a protein, but this does not mean that such RNAs do not contain information nor have function. There are two types of ncRNA categorized by their length: small ncRNAs and long ncRNAs (lncRNAs). Small ncRNAs including ribosomal RNA (rRNA), transfer RNA (tRNA), microRNA (miRNA), and piwi-interacting RNA (piRNA) have fewer than 50 nucleotides but are important regulators in gene expression. Apart from rRNA and tRNA, which act as the protein translational regulator, miRNA is able to mediate post-transcriptional gene silencing of complementary mRNAs by a process called RNA interference. In contrast with small ncRNA, lncRNA segments have longer sequences, typically more than 200 nucleotides. Having similar functions as miRNA, lncRNAs regulate gene expression such as protein translation and posttranscriptional silencing. In addition, lncRNA represses the translation of cis and trans gene by histone modifications or disrupting the miRNA regulation. CircRNAs are also a kind of ncRNAs, which structure is different from linear mRNAs. circRNAs have many types and complex functions, and have not been fully studied.