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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. They are rare or absent in most unicellular eukaryotes and prokaryotes, but present in the DNA of viruses that infect eukaryotic cells. In contrast to introns, the protein-coding segments of DNA are called [[exon]]s. 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 gene containing the protein to be purified must have its introns excised before being subcloned into a suitable plasmid. This is often done using a [[cDNA library]].