The future of Lance Armstrong's $500 million cancer charity is at stake as accusations of doping begin to catch up with the legendary cyclist.

Armstrong resigned his position as chairman of Livestrong Wednesday. The cyclist has raised about half-a-billion dollars since 1997 to help cancer patients.

His resignation comes on the heels of a report from the U.S. Anti-Doping Agency alleging that he used illegal performance-enhancing drugs.

The group banned the cyclist for life and ordered his titles stripped.

Several sponsors, including Nike, Anheuser-Busch, and Radio Shack, announced they're severing ties with Armstrong. Nike accuse Armstrong of misleading them for years.
    
Armstrong said his decision to leave Livestrong is meant to protect the foundation's reputation.

"I think over the last few weeks, Lance has been reading about how best to inoculate the mission of the Livestrong foundation and to protect the work that it does," Livestrong spokeswoman Katherine McLane said.

"He decided what would be best was for him to step down as chairman," she added. "He will continue to serve on our board, but chairmanship will be assumed by local philanthropist Jeff Garvey."

Meanwhile, Armstrong still maintains his innocence against the doping charges.