After capping, and as RNA polymerase II continues to transcribe the gene, RNA splicing begins. In this process, the introns are removed from the newly synthesized RNA and the exons are stitched together.
Each transcript ultimately receives a poly-A tail; in many cases, this happens after splicing, whereas in other cases, it occurs before the final splicing reactions have been completed.
Once a transcript has been spliced and its 5ʹ and 3ʹ ends have been modified, the RNA is now a functional mRNA molecule that can leave the nucleus and be translated into protein.
Before these steps are completed, the RNA transcript is known as a precursor-mRNA or pre-mRNA for short.