The recent discovery that small interfering RNAs (siRNAs) induce gene suppression in mammalian cells has sparked tremendous interest in using siRNA-based assays and high-throughput screens to study gene function. As a result, research programs at leading academic and commercial institutions have become a substantial and rapidly growing market for synthetic RNA. Important considerations in synthesizing RNA for biological gene function studies are sequence integrity, purity, scalability, and resistance to nucleases; ease of chemical modification, deprotection, and handling; and cost. Of the well-established RNA synthesis methods, 2'-ACE chemistry is the only one that meets all of these criteria. 2'-ACE technology employs a unique class of silyl ethers to protect the 5'-hydroxyl, in combination with an acid-labile orthoester protecting group on the 2'-hydroxyl (2'-ACE). 2'-ACE-protected phosphoramidite monomers are joined using standard solid-phase technology to achieve RNA synthesis at efficiencies rivaling those for DNA. This unit describes the synthesis of standard 5'-silyl-2'-ACE-protected phosphoramidites.