The design and construction of new proteins or enzymes with novel or desired functions, through the modification of amino acid sequences using recombinant DNA (deoxyribonucleic acid) technology. The sizes and three-dimensional conformations of protein molecules are also manipulated by protein engineering. The basic techniques of genetic engineering are used to alter the genes that encode proteins, generating proteins with novel activities or properties. Such manipulations are frequently used to discover structure-function relationships, as well as to alter the activity, stability, localization, and structure of proteins.