I think it’s referencing regulatory pathways. [m]essenger RNA is regulated by [mi]cro RNA which is regulated by [circ]ular RNA which is indirectly(?) regulated by [s]mall [n]uclear RNA.

Here’s a site that has helped me in the past, but I don’t know if it’s accurate or not, because I barely (not really) know what the heck I’m talking about.

Also some wiki links:

en.wikipedia.org/wiki/Messenger_RNA

en.wikipedia.org/wiki/MicroRNA

en.wikipedia.org/wiki/Circular_RNA

en.wikipedia.org/wiki/Small_nuclear_RNA