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The chemical composition of cells at the molecular level determines their growth, differentiation, structure, and function. Probing this composition is powerful because it provides invaluable insight into chemical processes inside cells and in certain cases allows disease diagnosis based on molecular profiles. However, many techniques analyze fixed cells or lysates of bulk populations, in which information about dynamics and cellular heterogeneity is lost. Recently, nucleic-acid-based probes have emerged as a promising platform for the detection of a wide variety of intracellular analytes in live cells with single-cell resolution. Recent advances in this field are described and common strategies for probe design, types of targets that can be identified, current limitations, and future directions are discussed., (© 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The ability to observe the dynamic of RNA in single living cells offers many exciting opportunities in biology and medicine. In the last few years, molecular beacons (MBs) have shown great potential in monitoring RNA synthesis, transportation, and localization with good sensitivity and selectivity. A hairpin structure probe, MB is a dual-labeled single stranded oligonucleotide that only fluoresces in the presence of target sequences. In this paper, the basic principle and design of MB will be described. The application of MB for RNA imaging in living cells will be reviewed. The limitations of MB for in vivo application will be identified. In the last section of the article, the efforts on designing better MBs for highly sensitive and selective RNA imaging will be discussed.