Project Summary Super-Resolution Fluorescence Microscopy (SRFM) has become an indispensable tool for scientists to probe discrete molecular proximities. Currently, systems in the Keck Center at the University of Virginia can only reach resolutions of ≥120 nm; to break that barrier for fixed or live cells, Leica STELLARIS 8 tauSTED microscopy can achieve ~30-50 nm resolution for fixed specimens. As a research and teaching institution in the highly competitive academic environment, it is essential to make these advanced technologies available for the biomedical sciences. A second driver for this application is the need to replace the existing and non-repairable 14-year-old Leica SP5x confocal microscopy system, purchased through the S10 program in 2007. The requested super-resolution tauSTED microscopy system will answer both needs and will benefit the basic and clinical sciences of this university; in particular, the 12 participating federally funded investigators (9 major and 3 minor users) of the Keck Center (KCCI). The higher quantum efficiency HyD (46%) and the new photon detection efficiency (PDE) detector (58%) provides a better option to excite the sample with less excitation intensity and detecting weaker photon levels in cells and tissues. This instrument is essential for our core investigator group to investigate the subcellular architecture and cell dynamics at nanoscale dimensions and will also be critical for training other investigators and students attending the fall semester microscopy course interested in the study of various biological applications including developmental, neuroscience and cell biology. The requested system will be housed in the internationally recognized W.M. Keck Center for Cellular Imaging (KCCI), which is particularly known for its advanced microscopy and molecular imaging. KCCI is a university-wide imaging facility, located within walking distance of all participating investigators from the School of Arts & Sciences, Engineering, and Medicine. KCCI is also recognized for its annual hands-on training FLIM & FRET workshop, held for the past 19 years at the University of Virginia. The requested super resolution tauSTED microscopy is an advanced technology, which will be housed in the Leica SP5x room (11'x14') and configured appropriately to meet the experimental conditions of users in this proposal and additional existing and future users of the KCCI imaging core. In summary, the requested instrument will provide the research community at the University of Virginia and beyond with a level of imaging capability that is not currently available and will enhance significantly ongoing and future research.