Introns are crucial because the protein repertoire or variety is greatly enhanced by alternative splicing in which introns take partly important roles. Alternative splicing is a controlled molecular mechanism producing multiple variant proteins from a single gene in a eukaryotic cell. One of the remarkable examples of the increasing protein repertoire by alternative splicing is the Drosophila Dscam gene, of which over 38000 isoforms can potentially be produced by alternative splicing. Pan et al. [27] have provided experimental evidence suggesting that approximately 95% of multiexon genes in the human genome may undergo alternative splicing. Furthermore, very short introns are selected against because a minimal length of intron is required for the splicing reaction [28].
