isomiRs (from iso- + miR) are miRNA sequences that have variations with respect to the reference sequence. The term was coined by Morin et al in 2008. It has been found that isomiR expression profiles can also exhibit race, population, and gender dependencies.
There are four main variation types:
*5 trimming—the 5 dicing site is upstream or downstream from the reference miRNA sequence
*3 trimming—the 3 dicing site is upstream or downstream from the reference miRNA sequence
*3 nucleotide addition—nucleotides added to the 3 end of the reference miRNA
*nucleotide substitution—nucleotides changes from the miRNA precursor. It is thought that may be similar process than post-transcriptional modifications.