mammalian AS

events frequently arise from the evolutionarily rapid loss or gain of exons from genomes (referred to as ‘genome-specific AS events’) [5–8]. How-Update TRENDS in Genetics Vol.21 No.2 February 2005 73occurs as a consequence of species-specific alternative splicing (AS) of conserved exons. Using a highly pre-dictive computational method, we estimate that O11% of human and mouse cassette alternative exons undergo skipping in one species but constitutively splicing in the other. These species-specific AS events are predicted to modify conserved domains in proteins more frequently than other classes of AS events. The results thus provide evidence that species-specific AS of conserved exons constitutes an additional potential source of complexity and species-specific differences between mammals. Alternative splicing (AS), the process by which exons in primary transcripts are spliced in different combinations, is a crucial step in the regulation of many vertebrate genes and is thought to have an important role in the generation of proteomic diversity [1–3]. Recent computational ana-lyses of AS have estimated that one-to-two-thirds of mammalian genes are alternatively spliced [4,5]. These and other studies have also provided evidence thatCorresponding author: Blencowe, B.J