The mirBase ID is the official miRBase name given to a mature miRNA sequence. There may be multiple miRBase IDs for a given mature miRNA sequence because many miRNAs are conserved across species (for example, several hundred human miRNAs have the identical sequence in common with the mouse species). The miRBase ID consist of a 3 letter species identifier, including the first letter of the genus and the first two letters of the species, followed by the expression miR, and then a numeric suffix, e.g.: hsa-miR-212. Since two distinct mature miRNAs can be derived from the different arms of the same stem-loop precursor, a -5p or -3p suffix specifies the particular arm of the stem from which they come, e.g.: hsa-miR-501-5p is derived from the 5' end of the stem, while hsa-miR-501-3p is derived from the 3' end. The miRNA gene and hairpin precursor locus name is given lower case: hsa-mir-212. If a mature miRNA is predicted to be expressed from more than one hairpin precursor, then a numeric suffix is added to the precursor name. For example, hsa-miR-101 is expressed from two precursor loci, hsa-mir-101-1 and hsa-mir-101-2. Closely related miRNAs are given lettered suffixes (hsa-miR-19a-3p and hsa-miR-19b-3p) and are expressed from similarly named precursors (hsa-mir-19a-3p and hsa-mir-19b-3p).
miRBase continues to re-annotate the miRNA sequences as more information becomes available. Updated versions of miRBase are released on a regular basis. At the time of print (June 2013), it was up to v20.