NGC 667 was a class M planet in or near Federation space in the 23rd century, and was the homeworld of a humanoid race called the Grennai. The planet was notable for the rings of radiation surrounding it, the effects of which the native life forms were immune, but were deadly to alien beings after prolonged exposure.
In 2265, Starfleet assigned a cultural observation team, including Doctors Campbell Roberts, Quentin Melander and Revati Jendra, to covertly study the Grennai. The team was pulled off the planet a year later after a mutated strain of Ametan rubeola eradicated an entire village.
Dr. Jendra returned to NGC 667 circa 2268 on a personal humanitarian mission. Captain James T. Kirk was later sent to NGC 667 to retrieve Jendra in that year, but later agreed to let her remain, reporting to Starfleet that she had died and was interred on the planet. (TOS - Constellations short story: "First, Do No Harm")