circrnas can be synthesized in vitro by circularization reaction using a transcription produced linear rna percussor, in which two flanking transposed halves of a t4td or anabaena derived split group i intron autocatalytically excise and the two flanking exons from t4td or anabaena ligate in tandem transesterification reactions to form circrnas8,17,18. it is important to note, however, that all of these pie strategies introduce exogenous sequences into circularized rnas, definitely from the retain of extraneous e1-e2 sequences.