Nucleic acid beacons for fluorescent in-situ hybridisation and chip technology

A nucleic acid capable of forming a hybrid with a target nucleic acid sequence and capable of forming a stem-loop structure if no hybrid is formed with the target sequence, said nucleic acid comprising

• (a) a nucleic acid portion comprising
• (a1) a sequence complementary to the target nucleic acid sequence, (a2) a pair of two complementary sequences capable of forming a stem,
• (b) an effector and an inhibitor, wherein the inhibitor inhibits the effector when the nucleic acid forms a stem-loop structure, and wherein the effector is active when the nucleic acid is not forming a stem-loop structure.