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Vaccination with in vitro transcribed RNA coding for tumor antigens is considered a promising approach for cancer immunotherapy and has already entered human clinical testing. One of the basic objectives for development of RNA as a drug is the optimization of immunobioavailability of the encoded antigen in vivo. By analyzing the effect of different synthetic 5′ mRNA cap analogs on the kinetics of the encoded protein, we found that m 2 7, 2′− O Gpp S pG (β-S-ARCA) phosphorothioate caps, in particular the D1 diastereoisomer, profoundly enhance RNA stability and translational efficiency in immature but not mature dendritic cells. Moreover, in vivo delivery of the antigen as β-S-ARCA (D1)-capped RNA species is superior for protein expression and for efficient priming and expansion of naïve antigen-specific T cells in mice. Our findings establish 5′ mRNA cap analogs as yet another module for tuning …