Following translation, many proteins undergo further modifications that can dramatically affect both their physical properties and biological function (Wold and Moldave, 1984; Freedman and Hawkins, 1985; Harding and Crabbe, 1992). These posttranslational modifications are essential to the vitality of all eukaryotic cells, including neurons. Techniques that identify amino acid residues in a given protein that are modified and assess the effect of eliminating a specific modification site on a protein’s function, both in vitro or in the context of cellular expression, are useful in studying posttranslational modifications. The more traditional biochemical and immunological methods of studying posttranslational modification are discussed elsewhere in this book. However, the resources required for these approaches may not always be available or may yield equivocal results. Thus, the powerful tools of molecular biology may provide a viable alternative for identifying sites of posttranslational modification.