Long peptides are frequently synthesized to study the protein-receptor binding. Usually, long peptides(100-200 residues), which is the size of a mean protein domain are expressed in Escherichia coli. But obtaining peptides sized 100–200 amino acids using chemical synthesis is much faster and cheaper than cloning and overexpressing in Escherichia coli. In addition, the resulting peptide is always correct. Chemical synthesis can be used to incorporate non-genetically encoded structures, such as D-amino acids, into the protein in a completely regular fashion. Synthetic peptides eliminate problems such as poor or no expression, cloning errors, tags like FLAG or 6-His, or the mis-translation of non-preferred codons in prokaryotic hosts. Artificial amino acids that have isosteric side chains can be used to investigate the functional importance of specific residues.