The process of cutting out introns from the primary transcript mRNA is called splicing. This is mediated by a complex molecular machinery called the spliceosome.

Here's a breakdown:

* Spliceosome: This is a large complex made up of proteins and small nuclear RNAs (snRNAs). It recognizes specific sequences at the ends of introns (called splice sites) and brings them together, allowing the intron to be looped out and excised.

* snRNAs: These RNAs (U1, U2, U4, U5, and U6) play crucial roles in recognizing splice sites and catalyzing the splicing reaction. They form base pairs with the intron and guide the spliceosome.

* Splicing Reaction: The spliceosome cleaves the intron at the 5' and 3' splice sites, and then ligates the two exons together. The excised intron is degraded.

So, in summary, the spliceosome, with the help of snRNAs, mediates the cutting of introns from the primary transcript mRNA.