A characteristic feature of elastomeric proteins is the presence of repeating motifs in the amino acid sequence which provide the freely mobile peptide chains that are the basis of their elasticity (see Chapter 17). These repetitive domains are highly ordered and mainly hydrophobic in nature and show an ability to selfassemble. The elastic domains are normally interspersed with short structured or ‘crystalline’ regions, which act as restraining domains. In some cases, the highly ordered ‘crystalline’ domains may also undergo conformational changes during self-assembly to allow subsequent precise intermolecular interactions. When isolated from the protein, these short amino acid sequences have been shown to have the ability to mimic many of the properties of the original protein, such as elastin or silk. More recently, amino acid sequences of both the ‘elastic’ and ‘crystalline’ domains have been probed by modern molecular biology techniques and synthetic polypeptides prepared. These peptides allow researchers to study the control mechanisms of their assembly and the basis of their elastomeric properties, knowledge of which can lead to the evolution of designer peptides for specialised applications. Further modification can be achieved by alteration of the amino acid sequence, or alteration of the amino acid side chains, generally the basic groups of lysine and arginine, and the carboxyl groups. Although several examples have been reported to date, only a few have found application; but this is an exciting field which is expanding rapidly with the advent of recombinant peptides.