Entry courtesy of
The National Human Genome Research Institute (NHGRI)

Exon

The region of a gene that contains the code for producing the gene's protein. Each exon codes for a specific portion of the complete protein. In some species (including humans), a gene's exons are separated by long regions of DNA (called introns or sometimes "junk DNA") that have no apparent function.

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Dr. John Carpten, of the National Human Genome Research Institute's Laboratory of Cancer Genetics, defines exon.