Provided herein are synthetic nucleic acid molecules known as loop-mediated riboregulators that have single-nucleotide polymorphism (SNP) sensitivity and ultralow OFF state signal levels. Loop-mediated riboregulators can activate or repress gene expression in response to trigger RNAs bearing completely arbitrary sequences. Also provided herein are methods of using such synthetic nucleic acid molecules for detecting the presence or absence of a particular target RNA in, for example, a biological sample.