IsomiR

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IsomiRs are variants of microRNAs (miRNAs). They come from the same miRNA precursor but have slightly different sequences. The term was first used in 2008 by Morin and colleagues. IsomiR expression can vary with race, population, and sex.

There are several ways isomiRs differ. They can have changes at the 5′ or 3′ ends or small substitutions inside the sequence. These differences arise because Drosha and Dicer—the enzymes that make miRNAs—cut at slightly different places, or because extra nucleotides are added to the 3′ end. Deep sequencing has shown that one miRNA precursor can produce many related isomiRs, which may target different genes or affect gene expression in different ways.

IsomiRs have been found across many species and are well described in human stem cells and brain tissue. They are real biological products, not artifacts from sample handling. Their levels can vary between populations, and some isomiRs change during development, suggesting they have important roles. The 3′ end additions (like adenylation or uridylation) can influence how stable the miRNA is and how it regulates target mRNAs.