fig1

Figure 1. Classification of RNAs. RNA can be classified as coding or non-coding RNA. Non-coding RNA can be separated into housekeeping ncRNAs and regulatory ncRNAs, which are the subject of this review. Regulatory ncRNAs can be separated according to their length, with small non-coding RNAs being less than 200 nucleotides and long non-coding RNAs greater than 200 nucleotides. snoRNAs and snRNAs are generally classified as sncRNAs but possess housekeeping functions. lncRNAs can be classified according to their relative location to protein-coding genes. The main types of small non-coding RNAs are also further classified here according to their function. Additionally, circRNAs and eRNAs are ncRNAs that can be of variable length, either greater or less than 200 nucleotides, so they do not fit into either length classification.