Significant efforts have been made to solve the ac4C-RNA synthesis problem but a satisfactory solution has not appeared. Using the N-cyanoethyl O-carbamate (N-ceoc) group, which can be removed under basic but non-nucleophilic conditions, for exo-amino protection, Meier and co-workers successfully synthesized ac4C-containing fragments of RNAs from the human 18S rRNA and tRNA Ser . Although a highly significant achievement, the method is still not ideal. Due to the use of a G phosphoramidite without amino protection, the method had to avoid the capping step, which may limit the length of RNAs that can be synthesized. In addition, the method uses a photocleavable linker to anchor RNA to solid support. Cleavage of RNA using UV may limit the scope of sensitive groups that can be incorporated into RNA, and the scope of sequences and the length of RNAs that can be synthesized. Other chemical methods that can potentially be used to synthesize sensitive RNAs such as ac4C-RNAs include those involving the use of palladium-deprotectable allyl and benzyl protecting groups, and the use of phosphoramidites without amino protection. However, these methods were only tested for DNA synthesis, and problems such as difficulty to remove palladium and branching may have discouraged researchers to test them for RNA synthesis. The fluoride-deprotectable ((t-butyl)(diphenyl)silyloxymethyl)benzoyl (SiOMB) and ((2-((triisopropylsilyl)oxy)benzyl)oxy)carbonyl (TBOC) groups were used for oligonucleotide synthesis including two reports related to RNA synthesis, but the success was limited. The oligonucleotides synthesized were short probably due to the instability of the protecting groups, and the method was not tested for the synthesis of epitranscriptomically modified sensitive RNAs including ac4C-RNAs. Some of the above methods as well as a few others were summarized in a recent review. Our research group reported the use of Dim and Dmoc groups, which can be deprotected under non-nucleophilic conditions by oxidation with NaIO4 followed by K2CO3 at room temperature, for sensitive oligonucleotide synthesis, but at this time, only sensitive DNAs have been synthesized. The study of these groups for RNA synthesis is still in progress. Besides chemical methods, enzymatic methods have been used for the synthesis of ac4C-RNAs as well, but these methods have limitations in efficiency, specificity, and sequence scope. Here, we report a sensitive RNA synthesis method using the 4-((t-butyldimethylsilyl)oxy)butanoyl (SoB)-based groups 4-((t-butyldimethylsilyl)oxy)-2-methoxybutanoyl (SoM) group for exo-amino protection and 4-((t-butyldimethylsilyl)oxy)-2-((aminophosphaneyl)oxy)butanoyl (SoA) group as linker. The traditional 2-cyanoethyl (2-CE), t-butyldimethylsilyl (TBS), 4,4′-dimethoxytrityl (DMTr) groups are used for phosphate, 2′-OH and 5′-OH protection, respectively. The phosphoramidite monomers are 1a-c as well as the commercially available 1d (Figure ). The SoA linker is introduced by phosphoramidite 1f. The RNAs synthesized can be represented by 2 (Scheme 1). Deprotection and cleavage are achieved by removing the CE groups with DBU, followed by treating with Et3N-HF, which simultaneously removes the SoM amino protecting groups and the TBS 2′-OH protecting groups, and cleaves the SoA linker (Scheme 1). The method can potentially be used for the synthesis of RNAs containing a wide ranges of sensitive group, but this paper is focused on ac4C. As a proof of concept, a 26-mer ac4C-containing RNA was synthesized. MALDI MS analysis indicated that the ac4C modification is completely stable under the deprotection and cleavage conditions.