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The exchange of a 3'-5'- for 2'-5' bond in the first step of lariat formation and the exchange of one 3'-5' phosphodiester for another in the second step of exon ligation are the two different reactions for subtracting the introns from nuclear precursors to messenger RNAs (pre-mRNAs) by the spliceosome. A study conducted to detect the stereochemical course for each step, utilizing the splicing substrate containing phosphothioate, showed that both steps occur as a single 'in-line' Sn2 nucleophilic substitution reaction, similar to the mechanism of group I self splicing introns. The reaction sites for second step of spliceosomal processing is similar to that of group I self splicing introns.

Introns have been considered examples of 'junk DNA,' a description for chromosomal sequences having no apparent function. This view was shattered by results of a cloning project aimed at sequencing the mouse UHG. UHG was found to be an example of an 'inside-out' gene wherein only introns encode function. The UHG structure is not believed to be unique, so further research could reveal other conserved intronic sequences.

In vitro formation of RNA which catalyzes bond forming reactions and the formation of a self-alkylating ribozyme shows that it is possible that in the beginning of evolution all reactions were catalyzed by RNA. The polyribonucleotides served as both the proteins and nucleic acids. RNA's with modified sugars and bases could have catalyzed the synthesis of active nucleosides.