Retro-genitor particles were a type of radiation that could stop Time Lords from regenerating. (AUDIO: X and the Daleks) They could prevent an individual from travelling in time without a time capsule. (AUDIO: The White Room)
Kotris and the Dalek time controller filled the atmosphere of Srangor with the particles so that the Time Lords would not discover their plans. (AUDIO: X and the Daleks) Kotris filled the two-year-old Molly O'Sullivan with the particles to further carry out their plan. The presence of the particles gave her dark eyes. (AUDIO: Tangled Web) The retro-genitor particles were removed from Molly after the timelines were reset after Straxus killed himself. (AUDIO: X and the Daleks) The Eighth Doctor later discover that the particles had reappeared in Molly. (AUDIO: The White Room)