Pre-mRNA molecules synthesized in the nucleus are subjected to specific RNA processing steps in both the nucleus and cytosol of cells. The RNA processing events in the nucleus include capping, splicing, and 3′-end processing (and, in some cases, RNA editing), while regulation of poly(A) tail length typically occurs in the cytosol. Each of these RNA processing steps can be regulated so as to generate multiple mRNA isoforms of a given gene. The depicted pre-mRNA molecule (boxes represent exons, while the line denotes introns) can be differentially spliced to generate mRNA isoforms that encode functionally distinct proteins. Pre-mRNA alternative splicing can also dictate isoform expression by regulating mRNA stability via inducing/blocking NMD. Differential isoform expression can further be achieved by regulating the mRNA export of specific isoforms. In the cytosol, mRNAs containing degradation signals can be selectively degraded, in turn leading to differential protein isoform expression. In each of the above scenarios, regulation is brought about by recognition of cis-acting elements in the pre-mRNA/mRNA by cognate trans-acting factors that promote (green ovals) or inhibit (red ovals) particular events. Light gray boxes indicate mRNAs subject to degradation. "AAAn" refers to the addition of a poly(A) tail.