Gelezen (species)

The Gelezens were a race of blue-skinned, fanged DNA vampires on the planet Gelezen who had exhausted the genetic material of the human DNA on which the template of their own race was based. One Dr Skelpa sought to replenish the DNA pool with fresh samples extracted from Martha Jones, having tracked her on Earth to Liverpool in 1963. Her DNA had been enhanced because of her experiences  travelling in time with the Doctor and potentially offered the clone-race the opportunity to live forever. Martha was ultimately responsible for destroying the time-field that protected the planet from outsiders and helped ensure their thousands of years of self-imposed isolationism. When the planet's barrier was destroyed, the Doctor freely offered a sample of his own DNA which effectively saved the Gelezens from becoming an extinct species and gave them new life. (DWA: Signs of Life)