NCI Dictionary of Genetics Terms

The NCI Dictionary of Genetics Terms contains technical definitions for more than 200 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.

One of two or more DNA sequences occurring at a particular gene locus. Typically one allele (“normal” DNA sequence) is common, and other alleles (mutations) are rare.