As a proof-of-concept study for a clinically relevant setting, CD123-directed mRNA-CAR T cells were produced from three cancer patients’ T cells in a lab scale using the optimized CAR-mRNA comprising the CleanCap as 5′ cap structure, the 5′ hAG UTR, the 3′ AES/mtRNR1 UTR, the 3′ poly(A) tail, as well as a full m1ψ-modification. Here, MT-RNR1 is linked to cancer.