An Intron is a non-protein-coding sequence of DNA that is initially copied into RNA but is cut out of the final RNA transcript (mature mRNA). They are found in most multicellular eukaryotes, but are rare in housekeeping genes and RNA-coding genes. They are also rare or absent in most unicellular eukaryotes and prokaryotes, but present in the DNA or RNA of viruses that infect eukaryotic cells. In contrast to introns, the protein-coding segments of DNA are called exons. The presence of introns in many genes can cause problems with protein purification when bacteria are used to propagate a protein, as they lack the machinery to excise introns from genes. The presence of these foreign structures causes the bacteria to recognize and degrade the protein, in a specific reaction that may be part of the bacterial defense against bacteriophage infection. The gene containing the protein to be purified must therefore have its introns excised before being subcloned into a suitable plasmid. This is often done using a cDNA library.
Molecular Cell Biology, 5th ed. Lodish et al. New York: W. H. Freeman and Co, 2004. pp 111-112.