Prior to the genomic sequencing era, the bible for those working in clinical genetics was McKusick’s Mendelian Inheritance in Man, which appeared in multiple editions between the 1960s and the late 1990s. This catalogue was organized according to general patterns of inheritance and focused on phenotypes. Beginning in the mid-1980s, it was replaced by Online Mendelian Inheritance in Man, a continuously updated catalogue documenting molecular relationships between genetic variation and phenotypic expression. This paper explores this resource’s evolution with attention to (...) how disease is distinguished from clinically irrelevant variation and how phenotypic similarities are captured in cases where there is no obvious genotypic association. It is argued that hybrid compromises are encoded into OMIM®; in addition to serving its key original purpose of being a diagnostic catalogue, it also began to record detectable variations in the genome even if they were not known to be associated with phenotypically visible disorders or even phenotypic variations. Although the impacts of geneticization have been well recognized, particularly in popular media, this example allows exploration of some of the historic, epistemic, and methodological causes that underlie tendencies toward disease geneticization in contemporary medicine, while highlighting that such gene-focused strategies may in fact be warranted in some contexts. (shrink)