TheRassilon Imprimatur, (PROSE:Interference - Book One) orRassilon imprimature, (PROSE:How did this creep get in here, Professor?) was abio-imprimatur engineered byRassilon intoTime Lordbiodata. It was responsible for mapping Time Lords onto thevortex as pure mathematics. (PROSE:Interference - Book One) According to a nightmare theFifth Doctor had, it was impossible to travel in aTARDIS without the Imprimatur. (PROSE:Divided Loyalties)

With the Imprimatur, Time Lords could put themselves into different time states just by thinking about the rightequations; atthe Academy, trainee Time Lords used it totransmigrate objects, though theCardinals said that abusing the Imprimatur in that way "played merry hell" with one's biodata. TheEighth Doctor used it to escape fromprison. (PROSE:Interference - Book One)

TheSixth Doctor claimed that the Rassilon Imprimatur was a symbiotic print in Time Lord physiology.Timeships likeTARDISes and theKartz-Reimer module needed to absorb the Rassilon Imprimatur through theirbriode nebuliser in order to prime them to allow anyone to use it. However, the Doctor lied about at least part of this description to trick theSontaranStike, who was listening nearby. (TV:The Two Doctors) TheSeventh Doctor later toldAce that the Rassilon imprimatur bonded him tohis TARDIS forever, but it did not enable truetelepathic communication between him and the ship; nor did it prevent anotherTime Lord from flying the TARDIS if theIdentity Recognition Module failed. (PROSE:How did this creep get in here, Professor?)

Bio-imprimaturs were responsible for protecting members of theGreat Houses fromdeeptime phenomena.PresidentUmbaste was launched into afugue state for a year after he dropped these bio-imprimaturs and approached thecaldera. (PROSE:The Book of the War)

During theLast Great Time War war,Rassilon changed the imprimatur's signature on newly minted Battle TARDISes that allowed him to take control of any TARDIS with his will. He did this withEris's Stealth Battle TARDIS moving it to the Dalek Null Space ship to destroy it. (AUDIO:Homecoming)