The Cancer Man would be more accurately named C.J. Spender. A close, personal friend of Fox Mulder's father, and co-conspirator in dealings with extra-terrestrials.

He is not named because he may have cancer due to his ever-present Morley cigarettes, but rather, was implicated somehow when Agent Scully discovered she had an inexplicable implant beneath the skin at the nape of her neck.

As soon as the implant was removed, she developed cancer, but when it was replaced, at Spender's suggestion, her illness disappeared.
...hence "the Cancer Man".

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