The T. Rowe Price U.S. Treasury Long-Term Index Fund Z Class (TRZUX) is a mutual fund that focuses on investing in long-term U.S. Treasury securities. This fund aims to provide investors with a low-cost, diversified option for accessing the U.S. Treasury market. With the "Z" class designation, this fund is typically only available to institutional investors and has a lower expense ratio compared to other share classes. Overall, TRZUX offers a conservative investment option for those seeking exposure to U.S. Treasury securities.