design: For the first library, we electroporated 50 ug of plasmid into 25 million 832/13 rat insulinoma cells for each biological replicate and lysed cells with TRIzol 24 hours later. After phase separation, we used the Direct-zol RNA Miniprep Kit to isolate total RNA. We enriched for mature mRNA transcripts with fragment(dT) beads then treated 2 ug of mRNA with DNase I to remove plasmid or genomic DNA contamination. We reverse transcribed 1 ug of mRNA using SuperScript III with a custom primer containing a 6-bp unique molecular identifier (UMI) targeting transcripts derived from the MPRA plasmids. We treated cDNA samples with DpnI to eliminate any residual plasmid DNA.