Symbiotic nucleus

Symbiotic nuclei were a part of Time Lord cells that allowed for safe time travel, (TV: The Two Doctors) as well as regeneration. (PROSE: Love and War, AUDIO: Panacea) They were what gave Time Lords their Time Sense, and were normally never seen anywhere in the universe except within a Time Lord. (GAME: Lost in Time)

The nuclei connected a Time Lord's mind to their TARDIS, meaning ships like the Master's TARDIS and the Doctor's TARDIS also possessed symbiotic nuclei. (AUDIO: The Evil One) TARDIS nuclei constantly updated the Matrix about their pilots. (AUDIO: The Brink of Death)

Some symbiotic connections between Time Lords and TARDISes could be stronger than the Time Lord realises; as was the case with the Doctor's first TARDIS, which the Doctor abandoned on Gallifrey. (AUDIO: Prisoners of Fate)

The symbiotic nuclei were sewn as recognition codes into a Time Lord's DNA upon admission at the Time Lord Academy. (PROSE: Interference - Book One, Christmas on a Rational Planet) They also enabled a Time Lord's ability to regenerate. (PROSE: Love and War) De-activating a Time Lord's symbiotic nuclei would prevent further regeneration. (AUDIO: Panacea)

It was believed that a Time Lord could only regenerate twelve times because the nuclei could only divide so much. The Valeyard, however, claimed this was imposed by Rassilon to prevent a thirteenth regeneration and keep Time Lords from immortality. (AUDIO: Trial of the Valeyard)

The Dogma Virus, which remained dormant in an infected Time Lord until regeneration, took hold in the event of regeneration by hijacking the Time Lord's symbiotic nuclei. (AUDIO: Panacea)

While recovering from regenerative collapse within his TARDIS, the Sixth Doctor used his symbiotic nuclei, combined with the TARDIS' telepathic circuits, to construct a synaptic interface. (AUDIO: Piece of Mind)

A temporal crisis which sapped power from the Doctor's TARDIS throughout their incarnations also caused them to "shed" their symbiotic nuclei at every space-time Waypoint they visited. This caused their memories to become unreliable and non-linear, causing time echoes across incarnations. (GAME: Lost in Time)