 Because they are independently stable, domains can be "swapped" by genetic engineering between one protein and another to make chimeric proteins.The concept of the domain was first proposed in 1973 by Wetlaufer after X-ray
crystallographic studies of hen lysozyme  and papain 
and by limited proteolysis studies of immunoglobulins.Wetlaufer defined domains as stable units of protein structure that could fold autonomously.