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Regeneration

"Cardinal Rassilon has been researching a method of regenerating diseased and decaying tissue. Via a series of permanently carried self-replicating biogenic molecules, the cells of a Gallifreyan body can be repaired, restored, and reorganized. This will result in a whole new physical form. The brain cells will similarly be rearranged, although to a lesser degree, thus ensuring the new incarnation will replicate the memories and personality of the former incarnation. Cardinal Rassilon intends for this method to only be used upon the Gallifreyan elite. He has also inputed a parameter of twelve regenerative cycles to avoid decaying biogenic molecules." - The Recorder, Zagreus (Doctor Who Audio Drama)