? Macromolecular structure determination using electron microscopy has become a useful research tool aimed at understanding biological processes, identifying conformational changes, and characterizing components in cellular pathways. Our long-term goal is to broaden the group of people able to determine macromolecular structures by providing integrated, easy to use, and efficient software for routine determination of three-dimensional macromolecular structures using electron microscopy. To accomplish these goals, we will develop an add-on package, Single Particle Tumbleweed, to our existing Tumbleweed electron microscopy software suite. Novel aspects of this software include a system to guide users through the reconstruction process, multiple efficiently coded algorithms for macromolecular structure determination, and integrated data analysis and visualization tools. The software will provide a consistent, easy to use graphical user interface; data management, analysis, and logging tools; and extensive user help including tutorials. During this development work we will also enhance Tumbleweed with additional structural analysis capabilities, increased I/O performance, more robust data management, and the ability to run on commodity hardware. The result of this Phase II SBIR will be a complete design and implementation of this software package allowing easy, efficient, and routine determination of macromolecular structures by biomedical research scientists and structural biologists. ? ?