The Rani's exile from Gallifrey

The First Rani was exiled from Gallifrey by the Time Lords after some of her lab mice, as a result of an experiment, grew to enormous size and ate the Lord President's pet cat. They also bit the President himself, triggering a regeneration. The Master, another exiled Time Lord, told the Rani that the Time Lords' made a cardinal error in exiling her. (TV: The Mark of the Rani)

The incident involving Ushas, later the Rani, had occurred following the departures of her fellow Deca members, the First Doctor and the Master. The President who lost his pet cat and a regeneration was Pandad IV, who also lost his Chancellor, Socra, a CIA agent who was devoured by the mice. (PROSE: The Legacy of Gallifrey) Feeling that she would never be forgiven for the incident, Ushas opted out of Time Lord society, becoming a renegade. She took a TARDIS and settled on Miasimia Goria, (PROSE: Divided Loyalties) under the name of "the Rani". (TV: The Mark of the Rani)

According to the Seventh Doctor, the Rani had nothing but contempt for all other Time Lords since her exile from Gallifrey. However, after hearing this, the Rani bluntly stated that her contempt started long before her exile. (TV: Time and the Rani)