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.