Working principles of common hybridization-based probes. (A) A schematic of linear antisense probes, whereby a dye-labeled recognition sequence binds to its complementary target illustrated in the same color.[9] (B) Linear FRET probes involve hybridization of two different linear antisense probes to adjacent regions of a target sequence, bringing a FRET pair into proximity that results in a fluorescence signal that can be monitored.[56] (C) In the off state, MBs have a duplexed stem region (illustrated in brown) that keeps a fluorophore and quencher in close proximity. Target binding to the loop region (illustrated in light blue) opens the stem region, separates fluorophore and quencher, and turns on fluorescence.[125] (D) Dual FRET beacons incorporate two MBs that can bind to adjacent regions of a target sequence. Similar to linear FRET probes, binding of the MBs to adjacent regions of a target sequence brings a FRET pair near one another, resulting in turn-on of FRET signal.[77]