With careful study of the array of RNAs produced by many different genes, it has become clear that alternative splicing is an important mechanism for the regulation of the synthesis of various biologically important proteins. The simplest form of such regulation results from the tissue- or temporal-specific splicing events that lead alternatively to functional (protein coding) and nonfunctional (not protein coding) RNAs, but other events, such as the inclusion or exclusion of particular alternative coding sequences within an mRNA are also potential regulatory events in that they eliminate the production of one type of protein and allow the production of another. In addition the production of RNAs with alternative protein coding capacities can be used to greatly increase the potential array of products from any particular gene. One advantage of such regulation by alternative splicing, as opposed to having multiple copies of related genes each with its own different array of transcriptional regulatory elements, is that particularly useful groupings of tissue- and temporal-specific enhancer elements need not be remodeled to serve the multiple different functions of a gene.