Fifth Doctor's regeneration

The Fifth Doctor underwent his regeneration into the Sixth Doctor within the control room of his TARDIS as a result of spectrox toxaemia contracted on Androzani Minor. (TV: The Caves of Androzani) This was the fifth regeneration of the 12-regeneration life cycle beginning with the First Doctor. (TV: The Time of the Doctor)

Legacy
The Doctor's friends relived this regeneration while going backwards through the Doctor's memories to learn about the Other. (PROSE: Lungbarrow)