Table 1

Nomenclature of microRNAs (miRNAs)

Nomenclature formatExplanation
mir or miRThe genes that encode miRNAs, primary transcripts of miRNAs, and stem-loop precursor miRNAs are all named using the italicised prefix ‘mir’. Mature miRNAs are named using the non-italicised prefix ‘miR’.
mir-X or miR-XWith the exception of a few early miRNAs (such as the let family), miRNAs are sequentially assigned unique identifying numbers, depending on when they are first published, for example, mir-31 or miR-31.
mir-Xa, mir-Xb, …Similar miRNA sequences within a species are distinguished by a lettered suffix, for example, mir-181a and mir-181b. However, this does not imply shared targets or functions.
mir-X-1, mir-X-2, …Identical miRNA sequences within a species are distinguished by a numerical suffix, for example, mir-7-1 (located on chromosome 9), mir-7-2 (located on chromosome 15) and mir-7-3 (located on chromosome 19), can all produce identical mature miRNAs.
miR-X or miR-X*Mature miRNAs can be formed from either arm of the stem–loop precursor miRNA (pre-miRNA; figure 1). In the majority of cases, one arm is much more commonly formed than the other. Previous convention was to name these strands according to their relative abundance, with the more common known as the ‘guide’ strand and taking the name miR-X (eg, miR-181a) and the less common form known as the ‘passenger’ strand and taking the name miR-X* (eg, miR-181a*).
miR-X-5p or miR-X-3pThe latest convention is to name mature miRNAs by the arm of the pre-miRNA from which they are derived, regardless of their abundance—those from the 5′ arm are named miR-X-5p and those from the 3′ arm as miR-X-3p. For example, miR-181a is now known as miR-181a-5p and miR-181a* is now known as miR-181a-3p. This avoids problems with the previous system if the abundance of each arm changes between tissues, developmental stages, or species.
hsa-miR-X, rno-miR-X, …All of the above naming conventions can be preceded by a three-letter code which identifies the species the miRNA is from, for example, hsa=homo sapiens (human); rno=rattus norvegicus (rat). Therefore, miR-181a-5p found in humans, could be represented as hsa-miR-181a-5p. Identical miRNAs are given the same number, regardless of species, for example, hsa-miR-21 and rno-miR-21.
  • The nomenclature of miRNAs has changed over time and can often be confusing. This table explains some of the most common nomenclature formats, both previous and current. X, in the above formatting examples, is used as a generic term to represent the unique identifying number given to individual miRNAs.