In the human genome, the majority of protein-encoding genes are interrupted by introns, which are removed from primary transcripts by a macromolecular enzyme known as the spliceosome. Spliceosomes can constitutively remove all the introns in a primary transcript to yield a fully spliced mRNA or alternatively splice primary transcripts leading to the production of many different mRNAs from one gene. This review examines how spliceosomes can recombine two primary transcripts in trans to reprogram messenger RNAs.