In this review, we describe technology and use of single-fluorophore imaging and detection in living cells with regard to application in systems biology and medicine. Because all biological reactions occur under aqueous conditions, the realization of single-fluorophore imaging using an optical microscope has led to the direct observation of biological molecules at work. Today, we can observe single molecules in individual living cells and even higher multicellular organisms. Using single-molecule imaging, we can determine the absolute values of kinetic and dynamic parameters of molecular reactions as a whole and during fluctuations and distribution. In addition, identification of the coordinate of single molecules has enabled super-localization techniques to virtually improve spatial resolution of optical microscopy. Single-molecule detection that depends on point detection instead of imaging is also useful in detecting concentrations, diffusive movements, and molecular interactions in living cells, especially in the cytoplasm. The precise and absolute values of positional, kinetic, and dynamic parameters that are determined by single-molecule imaging and detection in living cells constitute valuable data on unitary biological reactions, because they are obtained without destroying the integrity of complex cellular systems. Moreover, most parameters that are determined by single-molecule measurements can be substituted directly into equations that describe kinetic and dynamic models in systems biology and medicine.
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