Exon skipping in mammals
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  There are several different types of alternative splicing (AS) events,
  which can be classified into four main subgroups. The first type is
  exon skipping, in which a type of exon known as a cassette exon is
  spliced out of the transcript together with its flanking introns (see
  the figure, part a). Exon skipping accounts for nearly 40% of AS
  events in higher eukaryotes [17, 111] but is extremely rare in lower
  eukaryotes.