mRNA knockdown by single strand RNA is improved by chemical modifications.

While RNAi has traditionally relied on RNA duplexes, early evaluation of siRNAs demonstrated activity of the guide strand in the absence of the passenger strand. However, these single strands lacked the activity of duplex RNAs. Here, we report the systematic use of chemical modifications to optimize single-strand RNA (ssRNA)-mediated mRNA knockdown. We identify that 2'F ribose modifications coupled with 5'-end phosphorylation vastly improves ssRNA activity both in vitro and in vivo. The impact of specific chemical modifications on ssRNA activity implies an Ago-mediated mechanism but the hallmark mRNA cleavage sites were not observed which suggests ssRNA may operate through a mechanism beyond conventional Ago2 slicer activity. While currently less potent than duplex siRNAs, with additional chemical optimization and alternative routes of delivery, chemically modified ssRNAs could represent a powerful RNAi platform.