Gene expression involves a three-step process including transcribing precursor messenger RNAs (pre-mRNAs) from DNA templates by RNA polymerase II (Pol II), processing pre-mRNAs into mature mRNAs, and translating mature mRNAs into proteins (Bentley, 2014). The maturation of pre-mRNAs entails 5′ end capping, pre-mRNA splicing, 3′ end processing, and RNA modification (Bentley, 2014). Pre-mRNA splicing is a two-step transesterification reaction, catalyzed by the spliceosome in recognizing various sequence motifs in pre-mRNAs including 5′ and 3′ splice sites, branch points, polypyrimidine tracts, and other splicing regulatory elements (Herzel et al., 2017). For a long time, transcription and splicing steps were considered independent since both reactions could be reconstituted in vitro, respectively. However, new imaging and next-generation sequencing methods have revealed that both transcription and splicing occur simultaneously rather than sequentially in vivo, whereas previous studies in yeast and animals suggested that pre-mRNA splicing is mainly co-transcriptional instead of post-transcriptional (Bentley, 2014).