Recruitment model for cotranscriptional splicing of exon 2′/2. In this model, promoters 1A and 1B have differential abilities to bind SF2/ASF to the CTD of pol II. (A) In the 1A promoter, SF2/ASF is not recruited to the CTD; transcription of pre-mRNA exposes the distal 3′ss and allows for joining of exon 1A to produce a mature mRNA lacking exon 2′. (B) In the 1B promoter, SF2/ASF is recruited to the CTD of pol ll. The bound SF2/ASF blocks the distal 3′ss, favoring use of the proximal 3′ss, and results in the inclusion of exon 2′.