Use of 2-thiouridine (s2U) in an in vitro -synthesized mRNA, oligoribonucleotide or polyribonucleotide molecule to reduce the immunogenicity of said in vitro -synthesized mRNA, oligoribonucleotide or polyribonucleotide molecule, wherein the reduction in immunogenicity of said in vitro -synthesized mRNA, oligoribonucleotide or polyribonucleotide molecule comprising 2-thiouridine (s2U) is detectable by at least the following method, wherein the method comprises the steps of: a) contacting a murine or human dendritic cell with the in vitro -synthesized mRNA, oligoribonucleotide or polyribonucleotide molecule comprising 2-thiouridine (s2U), and, separately, contacting a murine or human dendritic cell with the in vitro -synthesized mRNA, oligoribonucleotide or polyribonucleotide molecule not comprising 2-thiouridine (s2U); and b) measuring the quantity of IL-12, IFN-α, TNF-α, RANTES, MIP-1α, MIP-1ß, IL-6, IFN-ß or IL-8 secreted in response to contacting the murine or human dendritic cell with the in vitro -synthesized mRNA, oligoribonucleotide, or polyribonucleotide molecule comprising 2-thiouridine (s2U), and, separately, measuring the quantity of IL-12, IFN-α, TNF-α, RANTES, MIP-1α, MIP-1ß, IL-6, IFN-ß or IL-8 secreted in response to contacting the murine or human dendritic cell with the in vitro -synthesized mRNA, oligoribonucleotide, or polyribonucleotide molecule not comprising 2-thiouridine (s2U), wherein the quantity of IL-12, IFN-α, TNF-α, RANTES, MIP-1α, MIP-1ß, IL-6, IFN-ß or IL-8 secreted from the murine or human dendritic cell in response to said contacting with the in vitro -synthesized mRNA, oligoribonucleotide, or polyribonucleotide molecule comprising 2-thiouridine (s2U) is less than the respective quantity of IL-12, IFN-α, TNF-α, RANTES, MIP-1α, MIP-1ß, IL-6, IFN-ß or IL-8 secreted from the murine or human dendritic cell in response to said contacting with the in vitro -synthesized counterpart mRNA, oligoribonucleotide, or polyribonucleotide molecule not comprising said 2-thiouridine (s2U). wherein the reduction in immunogenicity of said in vitro -synthesized mRNA, oligoribonucleotide or polyribonucleotide molecule comprising 2-thiouridine (s2U) is detectable by at least the following method, wherein the method comprises the steps of: a) contacting a murine or human dendritic cell with the in vitro -synthesized mRNA, oligoribonucleotide or polyribonucleotide molecule comprising 2-thiouridine (s2U), and, separately, contacting a murine or human dendritic cell with the in vitro -synthesized mRNA, oligoribonucleotide or polyribonucleotide molecule not comprising 2-thiouridine (s2U); and b) measuring the quantity of IL-12, IFN-α, TNF-α, RANTES, MIP-1α, MIP-1ß, IL-6, IFN-ß or IL-8 secreted in response to contacting the murine or human dendritic cell with the in vitro -synthesized mRNA, oligoribonucleotide, or polyribonucleotide molecule comprising 2-thiouridine (s2U), and, separately, measuring the quantity of IL-12, IFN-α, TNF-α, RANTES, MIP-1α, MIP-1ß, IL-6, IFN-ß or IL-8 secreted in response to contacting the murine or human dendritic cell with the in vitro -synthesized mRNA, oligoribonucleotide, or polyribonucleotide molecule not comprising 2-thiouridine (s2U), a) contacting a murine or human dendritic cell with the in vitro -synthesized mRNA, oligoribonucleotide or polyribonucleotide molecule comprising 2-thiouridine (s2U), and, separately, contacting a murine or human dendritic cell with the in vitro -synthesized mRNA, oligoribonucleotide or polyribonucleotide molecule not comprising 2-thiouridine (s2U); and b) measuring the quantity of IL-12, IFN-α, TNF-α, RANTES, MIP-1α, MIP-1ß, IL-6, IFN-ß or IL-8 secreted in response to contacting the murine or human dendritic cell with the in vitro -synthesized mRNA, oligoribonucleotide, or polyribonucleotide molecule comprising 2-thiouridine (s2U), and, separately, measuring the quantity of IL-12, IFN-α, TNF-α, RANTES, MIP-1α, MIP-1ß, IL-6, IFN-ß or IL-8 secreted in response to contacting the murine or human dendritic cell with the in vitro -synthesized mRNA, oligoribonucleotide, or polyribonucleotide molecule not comprising 2-thiouridine (s2U), wherein the quantity of IL-12, IFN-α, TNF-α, RANTES, MIP-1α, MIP-1ß, IL-6, IFN-ß or IL-8 secreted from the murine or human dendritic cell in response to said contacting with the in vitro -synthesized mRNA, oligoribonucleotide, or polyribonucleotide molecule comprising 2-thiouridine (s2U) is less than the respective quantity of IL-12, IFN-α, TNF-α, RANTES, MIP-1α, MIP-1ß, IL-6, IFN-ß or IL-8 secreted from the murine or human dendritic cell in response to said contacting with the in vitro -synthesized counterpart mRNA, oligoribonucleotide, or polyribonucleotide molecule not comprising said 2-thiouridine (s2U).