Gelezen (species)

The Gelezens were a race of blue-skinned, fanged, DNA vampire’s on the planet Gelezen who had exhausted the genetic material of the human DNA from which the template of their own race was based. They sought to replenish the DNA pool with fresh samples extracted from Martha Jones, DNA that had been enhanced because of her travelling experiences in time with the Doctor. Martha was ultimately responsible for destroying the time-field that protected the planet from outsiders and helped ensure their thousands of years of isolationism. When the planets barrier was destroyed, the Doctor freely offered up a sample of his own DNA which effectively saved the Gelezens from becoming an extinct species. (DWAM: Signs of Life)