Starbase 53 was a Federation Starfleet facility in operation in the 24th century. The starbase is located in the Largo star system. Section 31 operative Luther Sloan threatened to bring Dr. Julian Bashir to Starbase 53 for further interrogation during his holodeck encounter with the doctor. (DS9 episode: "Inquisition") In 2400 of the "War of the Prophets" alternate timeline, Captain Benjamin Sisko and the crew of the USS Defiant were brought to Starbase 53 after travelling to 2400 from 2374. (DS9 - Millennium novel: The War of the Prophets)