Symbiotic nuclei were a part of Time Lord cells that allowed for safe time travel. (TV: The Two Doctors) 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)
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) (PROSE: Christmas on a Rational Planet) They also enabled a Time Lord's ability to regenerate. (PROSE: Love and War) 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)