The intervening sequences or the introns do not code for any proteins. However, they are present in the gene separating the coding sequences from each other. They were discovered recently, a few decades back. Till now we only know a few types of intronic sequences and their locations. Some belong to nuclear pre-RNAs whereas others belong to organelle pre-RNAs. Complete maturation of RNA is tough to achieve without the removal of intronic sequences. Hence, without splicing of introns, RNA does not get translated into a protein. The pre-mRNA consists of an alternative arrangement of introns and exons. After a particular exonic sequence, an intronic sequence starts, followed by exons and introns again. Also, a mRNA consists of many introns with lengthy sequences covering the majority of the mRNA transcript. Thus, they take up a considerable length of the transcript, and thus they get spliced or excised. The exons, on the other hand, have specific sequences coding for amino acids. Hence they need to be ligated together.