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) Close proximity to someone who were infused with retro-genitor particles would cause their eyes to turn dark periodically. (AUDIO: Eyes of the Master)
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 was killed by the Dalek Time Controller. (AUDIO: X and the Daleks) The Eighth Doctor later discovered that the particles had reappeared in Molly. (AUDIO: The White Room)
the Reborn Master used the retro-genitor particles in Molly to resist and remove the Eminence's breath of forever in humans. (AUDIO: The Death of Hope)
The Dalek Time Controller inoculated himself with retro-genitor particles, (AUDIO: The Monster of Montmartre) which created a link between it and Molly O' Sullivan. This meant that if one of them felt pain, so did the other. (AUDIO: Eye of Darkness)