Cytosine base editors can be employed to disrupt the expression of inhibitory receptors in T-cells, and when combined with the introduction of chimeric antigen receptor (CAR) genes, can generate an improved drug product with tolerance to tumour-associated immunosuppressive signals. The manufacture of base-edited cellular therapies is limited by the large-scale production of mRNA encoding the base editor enzyme. Here, we present a novel method to produce mRNA encoding BE4max, that is efficient, scalable and financially viable.