The planet of Gelezen for thousands of years had an isolationist policy maintained by a protective time-field that surrounded the planet. The barrier was destroyed by Martha Jones when she was taken to the planet by a long distance teleport beam that first detected her in London in the 1960s. Martha was chosen for her DNA, which had been enhanced by travel with the Tenth Doctor through time. It was seen as a means of refreshing the corrupted human DNA material from which the Gelezens had been cloned. (COMIC: Signs of Life)

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