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- 1. Grandfather
The oldest known member of the Mulder family, Fox's grandfather was deceased by 1995 but was alive during Fox's childhood. He was known to have a poor stomach (TXF: "The Căluşari"), and also for slurping up his soup. (TXF: "Theef")
- It is possible that Fox was speaking of two separate grandfathers in those separate incidents. It is also possible that he was speaking sarcastically and making up those events. It was never established if either grandfather was William's father, a Mulder, or Teena's father, a Kuipers.
Main article: William Mulder
Elizabeth "Teena" Mulder, née Kuipers
Main article: Teena Mulder
Teena Mulder was born as "Elizabeth Kuipers" in 1941 and married William Mulder. She had two children, Fox and Samantha, and worked mainly as a homemaker in their Chilmark, Martha's Vineyard home. She committed suicide in 2001. (TXF: "Sein Und Zeit")
- Fox only implies information about his uncle and even the possibility that Fox ever had an uncle is never firmly established. It is also never established whether Fox's uncle was William's brother, a Mulder, or Teena's brother, a Kuipers.
Fox William Mulder
Main article: Fox Mulder
Fox Mulder, although biologically the son of the "Cigarette Smoking Man", was raised by William and his mother Teena, and became a Special Agent in the FBI. He was born on October 13, 1961. He investigated the X-Files because of the trauma of his sister's disappearance. (TXF: "Paper Clip") With his former FBI partner, Dana Scully, Fox had a child, William. (TXF: "William")
Samantha T. Mulder
Main article: Samantha Mulder
Samantha Mulder was born on January 22, 1964, and was abducted from her home on November 27, 1973. Her abduction traumatized her brother Fox and was connected to the founding of the Syndicate. She was later returned and raised by the Cigarette Smoking Man. She disappeared in 1979, and died in 1987. (TXF: "Closure"; "The Truth")
- The only evidence of Fox's wife was a ring he wore in the flashback episode "Travelers". There were no other mentions of a marriage.
Main article: William
- The possibility of whether William and Fox were biologically related or not is never established. However, the fact that William was considered to be Fox's son nevertheless makes this relationship technically that of father and son, meaning that William was therefore a member of the Mulder family.