1. Enzyme Nomenclature 1992;Nomenclature Committee of the International Union of Biochemistry and Molecular Biology,1992
2. Oligopeptidases, and the Emergence of the Prolyl Oligopeptidase Family
3. A. J. Barrett and H. Kirschke, this series, Vol. 80, p. 535.
4. C. G. Knight, this series, Vol. 248, Chapter 2.