Protein modules.

@article{Barn1991ProteinM,
  title={Protein modules.},
  author={Matilde Bar{\'o}n and David G Norman and Iain D. Campbell},
  journal={Trends in biochemical sciences},
  year={1991},
  volume={16 1},
  pages={
          13-7
        }
}
As the database of protein sequences grows it is becoming apparent that many proteins are constructed from relatively few modular units that appear many times. Determination of the three-dimensional structure of such modules by NMR has been possible due to their production in relatively large quantities by recombinant DNA techniques. The knowledge gained about the structure of individual modules can then be used to predict their properties in a variety of intact proteins.