 Twintrons were discovered by Donald W. Copertino and Richard B. Hallick as a group II intron within another group II intron in "Euglena" chloroplast genome.They found that splicing of both the internal and external introns occurs via lariat intermediates.Additionally, twintron splicing was found to proceed by a sequential pathway, the internal intron being removed prior to the excision of the external intron.