Chaz's Breast Removal Terrified Cher

With his girlfriend and his stepmom, Rep. Mary Bono Mack, in attendance, Chaz Bono premiered his documentary, Becoming Chaz, at Sundance over the weekend.

Oprah snapped up the rights to air the documentary on her OWN network before its Park City, Utah, debut. The movie chronicles Chaz's transition from female to male.

"I wanted to do this because I wanted to help people," he told E! Online. "And I knew that...I could put a face on this issue that people just don't understand because it's a hard one to understand. I wanted to be able to change people's hearts and minds on this."

In the film, Chaz is shown having his breast-removal surgery -- and Cher admits she was "terrified" to see her son after the procedure. Cher also says in the film she thinks her Chaz's father, Sonny Bono, would have supported their son's transition.

For more on the documentary, click here, and click here for more pics of Chaz and his girlfriend, Jennifer Elia.