One of the fundamental characteristics of multicellular organisms is their ability to generate diverse phenotypes among cells sharing a common genome. These specific cell phenotypes result from the regulated expression of protein isoforms that are structurally distinct, developmentally regulated and cell specific. The molecular mechanisms responsible for the generation of this protein diversity fall into two main classes: those that select a particular member of a multigene family for expression at a particular stage, cell type or physiological condition, as it occurs with the myosin heavy chain (1) and globin (2) gene families, and those that produce several proteins from a single gene. These latter mechanisms include DNA rearrangement (3), as it occurs in the immunoglobulin and T cell receptor gene families, and alternative pre-mRNA splicing that is very widespread and uses post-transcriptional steps to accomplish similar ends.