Studies of the roles of proteins in cells or tissues usually require that the cDNA encoding the protein be molecularly cloned and then expressed. We use these methods routinely not only in order to express proteins, but also to mutate them and to study the effects of the mutations on activity, and to identify other proteins to which they bind. We routinely use PCR methods for cloning and for site-directed mutagenesis, and the yeast two-hybrid screen to identify ligands of our cloned proteins or protein fragments. An example of the latter is shown in the section on Intermediate Filaments in projects: Contractile Apparatus and Costameres.