Messenger RNA (mRNA) is made up of exons and introns, that is, portions of genetic information that are "switched on" and "switched off", respectively. In order to reach maturation and be sent to the cellular "machine" that deals with protein synthesis, the non-coding fragments contained in the mRNA, the introns, need to be removed, whereas the coding sequences, the exons, have to be linked together. In humans, this "snipping and stitching" process, known as splicing, is governed by a huge machinery consisting of proteins and RNA, the spliceosome.