The alternative splicing events were classified into ten categories: alternative 3′ splice site, alternative 3′ UTR, alternative 5′ splice site, alternative 5′ UTR, alternative first exon, alternative last exon, mutually exclusive exon, skipped exon, tandem 3′ UTR, and tandem 5′ UTR. For more information on these events, refer to Wang et al.5 We then classify each primary exon into these classes based on all of the observed links of the primary exon. This means that a primary exon can be involved in multiple splicing events. From these classifications, we then calculate the proportion of each splicing class in the pool of significant primary exons. This method of classification was chosen because each link quantification is dependent on the quantification of all the other links that a primary exon makes.