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Table 3 Classification of human exon types based on RefSeq data

From: Noisy splicing, more than expression regulation, explains why some exons are subject to nonsense-mediated mRNA decay

Exon class Constitutive Alternative (Non-cassette) Alternative (Cassette)c Total
NMD-specifica 0 142 136 [82] 278
NMD-non-specifica 1,599 282 472 [251] 2,353
NonNMD-singleb 0 1697 4,110 [2,165] 5,807
NonNMD-multipleb 25,236 651 2,318 [1,137] 28,205
  1. Notes. NMD = nonsense-mediated mRNA decay. a the NMD-specific exons are only observed in NMD transcripts, and the left exons of NMD candidate genes are classified as NMD-nonspecific. b the NonNMD-single exons are the ones observed only in one transcript, and the left exons of Non-NMD candidates are called as NonNMD-multiple. c the numbers in the square brackets are the counts of cassette exons whose lengths are not divided exactly by three. Cassette exons are the alternative exons completely skipped or included in splicing isoforms.