There is some limited evidence from other sources that Mendelian-acting mutations give rise to MD. Attempts to fit morbid risk data to single major locus models have all been inconclusive (Gershon et al., 1976, Goldin et al., 1983, Price et al., 1985), as have been attempts to find markers that cosegregate with MD in a Mendelian inheritance pattern (Ashby and Crowe, 1978, Weitkamp et al., 1980, Wilson et al., 1989). A review of the online catalog of Mendelian disorders (OMIM) identified four single gene disorders in which MD is present as a clinical feature (Table 4). In addition (and not reported in the table), there are well-known relationships between MD and familial Cushing syndrome and Parkinson disease. The examples in Table 4 are rare, such as Perry syndrome, for which eight families are known worldwide, and typically present with additional phenotypes that would not lead them to be classified among the majority of cases of MD.Table 4Mendelian Conditions in which Major Depression Has Been Listed as a PhenotypeMIMNameClinical FeaturesPrevalenceInheritanceGene#168605Perry sydromeThe earliest and most prominent symptom may be MD not responsive
