Project Summary This proposed administrative supplement is for the purchase of a mass photometer to support research related to the parent grant as well as other NIH-funded research in the PI's department. This instrument will allow precise measurement of the formation of macromolecular complexes, providing information on the stoichiometry of the complex, the distribution of compositional states, and the affinity of complex formation. The instrument will support research related to multiple funded NIH awards, including the PI's research on CRISPR-Cas complexes involved in all stages of bacterial immunity. The instrument will be housed in the Iowa State Cryo-EM facility, where it will become part of the general sample screening workflow, while also allowing for more in-depth biophysical analysis. Facility users will be trained by the facility manager on all aspects of instrument operation and data analysis. The Cryo-EM Facility is administered by the Office of Biotechnology, which is under the direction of the Office of the Vice President for Research at Iowa State. The two offices will provide funds to offset the cost of the instrumentation and to purchase a service contract. User fees will be collected to provide funding for recurring instrument costs. The instrument fills a need for a biophysical technique to measure complex formation that is not possible using existing equipment on the Iowa State campus.