NCI Dictionary of Genetics Terms

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Welcome to the NCI Dictionary of Genetics Terms, which contains technical definitions for more than 150 terms related to genetics. These definitions were developed by the PDQ® Cancer Genetics Editorial Board to support the evidence-based, peer-reviewed PDQ cancer genetics information summaries.

simplex case
(SIM-plex kays)
A single, isolated occurrence of an inherited condition in a family. Simplex cases can result from certain inheritance patterns or be caused by de novo mutations or reduced penetrance within the family. It can also result from nongenetic causes such as adoption or alternate paternity.