design: Aliquots from in vitro trans-splicing reactions incubated for 22 h were prepared for sequencing as described in the in vitro selection procedure. In short, reactions were quenched using 4 L of 50 mM EDTA, and products were ethanol precipitated, and reverse transcribed using Superscript III reverse transcriptase (Invitrogen, Waltham, MA, USA). The resulting cDNA was PCR-amplified and subjected to two additional rounds of PCR that added Illumina adapters and Illumina barcodes (Illumina, San Diego, CA, USA). The final product was submitted for HTS using Illumina Novaseq X Plus (UC San Diego IGM Genomics Center).