Used in solid-phase oligonucleotide synthesis, particularly for the preparation of modified RNA strands. The compound acts as a protected cytidine phosphoramidite building block, enabling site-specific incorporation of cytidine into growing RNA sequences. The 2′-thiomorpholine-4-thiocarbonyl group provides enhanced 2′-OH selectivity during coupling, improving efficiency and reducing side reactions. The 4,4′-dimethoxytrityl group at the 5′-end ensures reversible protection for stepwise chain elongation, while the 2-cyanoethyl phosphoramidite moiety allows for mild deprotection and efficient coupling under standard synthesis conditions. This derivative is especially valuable in the synthesis of antisense oligonucleotides, siRNA, and other therapeutic RNA molecules where high fidelity and yield are critical.