The incredible diversity of life on this planet is made possible by proteins, all of which may be represented as linear heteropolymers of the same 20 amino acids, acting in specific catalytic and/or structural roles. The number of possible linear combinations of these amino acids in the average-sized protein chain exceeds the extimated number of atoms in the universe, and is thus fully compatible with the diversity of life. In the extreme, examples abound of single amino acid changes in a single protein in an organism which carry fatal consequences. Members of the Structural Biology Group in the Deaprtment of Biological Sciences, each working in their own system, seek to understand the details of both normal and mutant protein interactions, through a combination of techniques including X-ray crystallography, kinetic analysis, site-directed mutagenesis, and chemical sequence analysis.