The University of North Carolina at Chapel Hill is the coordinating center and an enrollment site for the registry. There are seven other enrollment sites: Ball Memorial Hospital, Muncie, IN; Duke University, Durham, NC; Hospital for Special Surgery, New York, NY; Johns Hopkins University, Baltimore, MD; Morehouse School of Medicine, Atlanta, GA; University of Texas Health Science Center, San Antonio, TX; and University of Utah Health Science Center, Salt Lake City, UT.