MetamorphoSys, free Java software, is distributed with the UMLS to assist users in producing subsets of the Metathesaurus. It includes some predefined filters for generating subsets that include SNOMED CT. Users can modify filters and defaults to generate a desired subset of the Metathesaurus, or write their own custom filters. Any output from MetamorphoSys includes some basic UMLS Metathesaurus data, such as NLM Concept Unique Identifiers (CUIs) as well as Lexical Unique Identifiers (LUIs), and String Unique Identifiers (SUIs). The UMLS RRF Browser, also free Java software distributed with the UMLS content, allows users to browse any subset output by MetamorphoSys.