The advent of powerful molecular techniques to study protein structure and function allows the modification of the codmg region of genes to produce proteins with altered structures that can be studied for changes in biochemical properties. The most common way to do structure/function studies is by site-directed mutagenesis, in which a particular residue in a protein is selected and systematically altered. Because of the labor-intensive nature of this approach, it is necessary to limit the scope of such studies to relatively few residues of a protein. Predictions of which residues are of functional importance are usually based solely on homology to related proteins and are unreliable, because sequence homology is a reflection of evolutionary relatedness as well as function. Also, the high degree of sequence homology present in many gene families makes it difficult to limit the scope of such studies. Another disadvantage of site-directed approaches is that often mutations introduced at critical residues destroy function, which can be caused by many trivial effects unrelated to the question bemg tested, such as improper protein folding, and so on.