Advances in molecular biology, organic chemistry, and materials science have recently created several new classes of fluorescent probes for imaging in cell biology. The possibilities are endless: the probes can be used to study proteins in live or fixed specimens; in vitro or in vivo; to study localization or activity, or modify function; and by light and electron microcopy. With the emergence of specialized microscopy units at most universities and research centers, the use of these techniques is well within reach for a broad research community. However, the fluorescent toolbox has become very complex and each tool has specific requirements and pros and cons for different applications. The aim of this project is to develop, validate and implement a probe for combined fluorescence live-cell imaging and electron microscopy of proteins of interest. The combinatorial probe will be based on a genetically-encoded tag consisting of a fluorescent protein (highly suitable for live cell imaging) and horseradish peroxidase to visualize proteins by electron microscopy at high resolution with high quality preservation of the ultrastructure. In addition, we will explore the general application of a recently introduced manner of increasing resolution using light microscopy (photoactivatable localization microscopy; PALM). While the focus of this project proposal is to create probes for general application in cell biology, initially these probes will be implemented in my studies that aim to resolve the role of cell-cell contacts in regulating beta cell proliferation.