First appearance of Alex Krycek, played by Nicholas Lea. Lea assumed a different role earlier in the series, in Gender Bender (1994).
While performing an autopsy, Scully receives a phone call from Mulder who states that his name is "George Hale". George Hale was the visionary behind the Palomar Observatory in California. Mulder explained this to Scully in the past episode Little Green Men (1994), where he also used it as an alias for his flight to Puerto Rico.
The character "X" was originally intended to be a woman; Natalija Nogulich even filmed a scene in the role, but she was recast with Steven Williams.
First on-screen appearance of "Mr X".
This is Howard Gordon's first solo writing effort for the series. Gordon was inspired to write the episode following a two-week bout of insomnia.