During the last years of his work on the X-Files, Mulder was even forced to doubt that Bill was his true father. He is led to consider the possibility that his mother had had an affair with the Smoking Man, a connection which may have resulted in the birth of either Samantha or Mulder himself. The possibility was hinted at later in the series, and Jeffrey Spender, who certainly was the Smoking Man's son, said that Mulder was his half-brother. In the ninth season, "William" explains that Spender and Mulder have very similar DNA, providing strong evidence that they do have the same father.