Fig. 1

Alternative splicing and alternative back-splicing. Linear splicing (A) of pre-mRNA removes introns and joins exons to produce mRNA. While constitutive splicing arranges exons and introns unbiasedly, alternative splicing (B) selectively assembles exons and introns to engender multiple mRNA variants corresponding to the same pre-mRNA. Back-splicing (C) shares the same splicing machinery with linear splicing and relies on the inverted complementary sequences (ICSs) in the flanking introns. Similarly, alternative back splicing (D) produces circRNA variants. CircRNA, circular RNA; mRNA, messenger RNA