In The X-Files Season 1 finale "The Erlenmeyer Flask", Mulder and Scully learn the following:
DEEP THROAT: Doctor Berube was killed because his work was too successful. You're standing in the room where the first DNA transplant took, the first human-alien hybrid was created.
However, in the Season 6 episode "Two Fathers", we have the following:
MULDER: I believe Cassandra Spender represents the final stages of these experiments — that she may very well be the first successful alien-human hybrid.
And in the sequel episode "One Son":
MULDER: She's the first, isn't she? She's the first successful alien-human hybrid.
Why would Mulder ignore the existence of the previous group of
successful hybrids and believe that Cassandra was the first?