A segment of deoxyribonucleic acid (DNA) transcribed into ribonucleic acid (RNA) as part of a longer strand of RNA, called heterogeneous nuclear RNA (hnRNA), but that does not survive the processes of messenger RNA (mRNA) maturation. Hence, genes are split into portions, termed exons, that appear in mRNAs or in structural RNAs, and introns. During the maturation of pre-mRNA to mature mRNA, a series of control processes occur that specify and narrow down the set of mRNAs that become functionally active in a given cell. These processes multiply the species of transcripts through the differential combination of exons and the inclusion of parts of introns into the functional mRNAs; thus, several mRNA products can be generated from a single gene in a controlled manner. For example, certain exons may be included or spliced out during mRNA maturation (alternative splicing of the same hnRNA molecule), or two different hnRNA molecules may form during the splicing reaction of one mRNA transcript, a process termed trans-splicing.