A small conditional RNA (scRNA) is a small RNA molecule or complex (typically less than approximately 100 nt) engineered to interact and change conformation conditionally in response to cognate molecular inputs so as to perform signal transduction in vitro, in situ, or in vivo. In the absence of cognate input molecules, scRNAs are engineered to coexist metastably or stably without interacting. Detection of the cognate inputs initiates downstream conformational changes of one or more scRNAs leading to generation of the desired output signal. The output signal may be intended to read out the state of endogenous biological circuitry (e.g., mapping gene expression for biological research or medical diagnosis), or to regulate the state of endogenous biological circuitry (e.g., perturbing gene expression for biological research or medical treatment). scRNA sequences can be programmed to recognize different inputs or to activate different outputs, achieving even single-nucleotide sequence selectivity. scRNA signal transduction exploits principles from the emerging disciplines of dynamic RNA nanotechnology, molecular programming, and synthetic biology. (en)