It refers to the arm the mature sequence comes from. In the beginning people used the miR and miR* notation for guiding (functional) mature miRNA (miR) and passenger (non-functional) mature miRNA (miR*). But then it was realized, that both arms can be functional, or they can share the function by switching between the 'left' and the 'right' arm, known as 'microRNA arm-switching'. That can happen between different tissues. In one tissue, the 'left' mature is expressed (-5p) and in another the 'right' one (-3p). miR and miR* would be confusing, since both are functional, but you cannot call both miR*, so researchers changed the notation to -5p and -3p.

I hope that helps a bit. :)

It refers to the arm the mature sequence comes from. In the beginning people used the miR and miR* notation for guiding (functional) mature miRNA (miR) and passenger (non-functional) mature miRNA (miR*). But then it was realized, that both arms can be functional, or they can share the function by switching between the 'left' and the 'right' arm, known as 'microRNA arm-switching'. That can happen between different tissues. In one tissue, the 'left' mature is expressed (-5p) and in another the 'right' one (-3p). miR and miR* would be confusing, since both are functional, but you cannot call both miR*, so researchers changed the notation to -5p and -3p.

I hope that helps a bit. :)

That's right! For any further information on miRNA nomenclature you can check miRbase.org: http://www.mirbase.org/help/nomenclature.shtml. I found it really helpful

All very interesting and clear replies however, I have a quick follow up question. Why do some miRNAs not have the 3p or 5p annotation? For example, hsa-miR-3687? It's entry in the mirBase can be found here:

http://www.mirbase.org/cgi-bin/mirna_entry.pl?acc=MI0016088

Thank you for the reply!

@ David MacIntyre: because nomenclature in miRBase is inconsistent.

@ David Langenberger: there are examples where arm-switching occurs, yes and it has been suggested to treat those miRNAs as co-matures. However, the vast majority of miRNAs is distinguishable in mature/star species.

When mature miRNA is synthesized from the arm that  ends at 5' end known as 5P and similarly when synthesized from the arm that ends at 3' end known as 3P.

In this case lat-7a-5p arises from the 5′ arm of the lat-7a hairpin, and lat-7a- 3p arises from the 3′ arm. also with “*” tells us that lat-7a* is considered a “minor” product. That means lat-7a* is found in the cell at lower concentration than lat-7a. It is often inferred that lat-7a* is non-functional, and you’ve probably noticed that lat* sequences in general magically disappear in most pictures of miRNA biogenesis, while the dominant arm is magically incorporated into the RISC complex. An older convention sometimes used lat-7a-s and lat-7a-as.

where can I search miRNA* sequence? Although i normally used miRbase website for miRNA sequence, I am not clear which is miRNA* (complementary stand).

Thank you.