The Typhon sector is an area of space on the coreward edge of the Federation in the 24th century, beyond which lies the Typhon Expanse. It is within close proximity to the Romulan Star Empire, Sector 221-G and the former Thallonian Empire. The Typhon sector also lies on the so-called Borg vector, the most frequent route of Borg incursions into the Alpha Quadrant. (Reference book: Star Trek: Star Charts; TNG: "Cause and Effect"; NF novels: House of Cards, et. al; Star Trek: First Contact)
By the 2370s, Starfleet maintained at least three facilities in or near the sector: Deep Space 4, Deep Space 5 and Starbase 185. (Reference book: Star Trek: Star Charts; Typhon Station)
In 2373, this sector was the rendezvous point for a Starfleet task force led by Vice Admiral Hayes, and the location of the initial engagement with a Borg cube enroute to Earth. The fleet failed to stop the cube in the sector and fought it all the way to Sector 001. (Star Trek: First Contact)