H. Gu, Y. Perl,  J. Geller, M. Halper, L. Liu and J.J. Cimino, Representing the UMLS as an OODB: Modelling Issues and Advantages. Journal of the American Medical Informatics Association, 7(1), January 2000, pp.66-80

Abstract: Objective: The Unified Medical Language System (UMLS) designed by NLM combines many well established authoritative medical informatics terminologies in one knowledge representation system. Such a resource is very valuable to the healthcare community and industry. However, the UMLS is very large and complex and poses serious comprehension problems for users and maintenance personnel. We present a representation to support the comprehension and navigation of the UMLS.