text: RNA activation (RNAa) is a small RNA-guided and Argonaute (Ago)-dependent gene regulation phenomenon in which promoter-targeted short double-stranded RNAs (dsRNAs) induce target gene expression at the transcriptional/epigenetic level. RNAa was first reported in a 2006 PNAS paper by Li et al. who also coined the term "RNAa" as a contrast to RNA interference (RNAi) to describe such gene activation phenomenon. dsRNAs that trigger RNAa have been termed small activating RNA (saRNA). Since the initial discovery of RNAa in human cells, many other groups have made similar observations in different mammalian species including human, non-human primates, rat and mice, plant  and C. elegans, suggesting that RNAa is an evolutionarily conserved mechanism of gene regulation.
  RNAa can be generally classified into two categories: exogenous and endogenous. Exogenous RNAa is triggered by artificially designed saRNAs which target non-coding sequences such as the promoter and the 3’ terminus  of a gene and these saRNAs can be chemically synthesized  or expressed as short hairpin RNA (shRNA). Whereas for endogenous RNAa, upregulation of gene expression is guided by naturally occurring endogenous small RNAs such as miRNA in mammalian cells  and C. elegans, and 22G RNA in C. elegans.