Helena was a planet that was home to a Federation colony consisting of varied species in the mid 24th century. In 2370, it was turned over by the Federation to the Cardassian Union, and placed within the Demilitarized Zone between the two powers. The colonists, known as Helenites, chose to give up their Federation citizenship.
The planet was settled by mixed-race colonists who were trying to escape discrimination in the rest of the Federation. Most Helenites were genetically bred hybrids made up of various races. Their whole social structure was based on genetics; the more unique the genetic heritage, the higher the social status. Non-mixed individuals were also present, and while at times they were not allowed in certain places, they still had some of the same luxuries as their mixed counterparts.
In 2371, a plague struck Helena, similar to one that had hit Archaria III and Bajor. The Cardassians threatened to sterilize the entire planet to prevent the disease from spreading. In an effort to alleiviate the illness, Maquis fighter intercepted a shuttle being piloted by Lieutenant Thomas Riker of the USS Gandhi carrying medical supplies from Outpost Sierra III. Riker was taken to Helena and, witnessing the situation there, decided to join the Maquis and use the medical supplies to help the colonists. (TNG - Double Helix novel: Quarantine; TNG - Slings and Arrows eBook: A Weary Life)