Alternative pre-mRNA splicing involves editing of a gene to generate a number of different mRNAs and proteins. It provides a mechanism for only 20,000 genes to generate hundreds of thousands of proteins. Like other proteins, it is estimated that 50% of G protein-coupled receptors undergo alternative splicing. While most commonly involving either the N-terminus or C-terminus, some variants have modifications in the interior of the receptor. Alternative splicing generates functionally distinct variants, due to an intrinsic difference in transduction or location. These features are well illustrated by the mu opioid receptor gene, OPRM1, which undergoes extensive alternative splicing.