Proteins are made of several tens to several hundreds amino acids. Sequences of amino acids are biologically determined by nucleotide sequences. The amino acid sequence, in turn, determines the protein structure that is responsible for catalysis. Artificially mutated protein can be made by modification of the gene sequence followed by the expression of the modified gene in Escherichia coli. Modified proteins are used in research on function and structure of the protein. The technique is called "Protein engineering".