Breen system
The Breen system was an inhabited planetary system located inside Breen space in the Alpha Quadrant. It was a single star system containing the planet Breen, which was the homeworld of the Breen, a warp-capable humanoid species. This was the capital system of the Breen Confederacy and, in 2375, was allied with the Dominion. (DS9: "For the Uniform", "Strange Bedfellows")
Stellar cartography
In 2399, the location of Breen was labeled in a Federation star chart that was in Fleet Admiral Kirsten Clancy's office at Starfleet Headquarters. This system was in neutral space. (PIC: "Maps and Legends")
Appendices
Background information
The primary of the Breen system was classified as a M-class star by the Star Trek: Star Charts (p. 36) and Stellar Cartography: The Starfleet Reference Library ("Federation Historical Highlights, 2161-2385").
Apocrypha
In the Star Trek Roleplaying Game book, Worlds, the star system of the Breen homeworld was identified as the Beta Epsilon Venturi system, a binary star system.