An intron was part of the DNA in a gene that had no function for coding in protein synthesis.
In 2370, while attempting to cure Lieutenant Reginald Barclay of Urodelan flu, Dr. Beverly Crusher used a synthetic T-cell to activate the gene that would fight off the infection. The result was the mutation and activation of Barclay's dormant genes, including his latent introns, thereby creating Barclay's Protomorphosis Syndrome. (TNG: "Genesis")