This website is based on a definitive work on genetic disorders, Victor McKusick’s Mendelian Inheritance in Man. But whereas the book is now four years old, OMIM is updated daily—as of January 2003 it included 14 120 references, gaining more than 60 a month and revising a further 500.

The site’s authors have reviewed the literature to provide a series of well-referenced summaries of clinical and molecular features, classified on the basis of mode of inheritance. At present, the majority are defective genes or loci that have been established beyond doubt as causing a single disorder with a sole means of transmission (eg the expanded trinucleotide repeat of Huntington’s disease). The remaining entries are currently of uncertain inheritance, either because the same condition can result from more than one genetic defect (such as Alzheimer’s disease), or because the phenotype may overlap with another.

OMIM is best accessed through the linked Entrez system (familiar as the home of PubMed), integrating it such that a few clicks will generate literature citations, DNA sequence data, or chromosome maps. The site is best at providing an initial overview of a disease or mutation. Some conditions have generated large entries, and are helpfully linked to clinical synopses and to “Mini-MIM”—a summary of their basic details. Readers should, however, remember that OMIM is a secondary source of information—it only reviews the literature and thus lags behind the very latest findings. In other instances, the limitations of the curators—geneticists but not necessarily super-specialists—become apparent, with an over emphasis on historical details and potential omission of relevant information. Searching for “spinocerebellar ataxia” generates 78 references, but omits Machado-Joseph disease (SCA3; listed only by its eponym).

Providing the caveats about the information it generates are heeded—more details and, especially, a balanced expert overview will be found by pairing an OMIM search with the reading of a recent review—OMIM is an authoritative, easy to use, and continually updated site, and an excellent first source of information for neurological clinicians and researchers.