The X-Files

The X-Files was a television programme from Earth, about FBI agents who investigated alien and paranormal events.

An older version of Peri Brown once posed as a secret agent to her younger self and claimed to be working for the X-Files. When she tried the ruse on the Fifth Doctor, with whom her younger self was travelling, the Doctor replied that while he had enjoyed the programme, it was never as good after David Duchovny left. (CC: Peri and the Piscon Paradox)

Among a long list of actions the Xlanthi considered to be a violation of law and punishable by a brutal death was "fancying Scully out of The X-Files." (BNA: Beige Planet Mars)

Behind the scenes

 * While recording the DVD commentary for Spearhead from Space, Nicholas Courtney was amused by the fact that visible on a shelf behind the Brigadier's desk was a binder labelled "X" - in other words, "the X file."