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We also observed some toxicity in our initial experiments with mRNA. However, incorporating the proper 5’ UTR and 3’ UTR sequences into the template used for in vitro transcription quickly resolved it. Another key factor was the purity of the mRNA. Ensure that the 260/280 OD ratio is between 1.8 and 2.1. The quality of the mRNA can also be determined by running a small sample on a gel to check for the proper size. In some scenarios, it might be best to also incorporate chemically modified nucleotides to the transcription reaction. Another reason for toxicity may be a result of the cells themselves; if the cells are at too low a density then there will be significant toxicity (ideal viable cell density on the day of transfection is between 70-90% confluence).