Complex sentence: Instead of expanding the genetic code, other researchers have investigated the structure and function of proteins by reducing the normal set of 20 amino acids, that is, by generating proteins where certain groups of amino acids may be substituted with a single amino acid.
Split into simpler sentences: Instead of expanding the genetic code, other researchers have investigated the structure and function of proteins by reducing the normal set of 20 amino acids. Limited protein sequence libraries are made by generating proteins where certain groups of amino acids may be substituted with a single amino acid.