Do concept unique identifiers (CUIs) in UMLS have a hierarchy?
As far as I know, there is a hierarchy, but it is based on the original sources (ie not created by UMLS). For example, SNOMED concepts belong to a hierarchy. UMLS records this hierarchy in the mrhier table. It is probably best to consider the hierarchy based on the source's code, and not on the UMLS concept ID, as the mapping between UMLS CUIs and codes are often very rough. Source: https://www.ncbi.nlm.nih.gov/books/NBK9676/