The celebrity cast of ABC's "Dancing With the Stars" season 17 is getting ready for tonight's premiere, including two participants who are maintaining a strong attitude despite their battle with serious illnesses.

Actress Valerie Harper, who has been diagnosed with terminal brain cancer that appears to have retreated for now, told "Good Morning America" correspondent Melissa Rycroft that, physically, she's feeling very well.

At 74, Harper is the oldest celebrity competitor in the cast. She says she has a strained knee for which she's receiving therapy, but added that she "wouldn't be doing this if it was endangering me in any way."

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"And I think it was a great opportunity to get out there and say, you know, 'I have cancer … Keep living your life. Keep moving and you can win. You can extend the time, at least,'" she said.

For TV personality Jack Osbourne, who has multiple sclerosis, participation on the program will be significant in several ways.

"One thing I hope to accomplish is, I guess, mainly, just to kind of put it out there, in all seriousness, that when you get diagnosed with MS that your life doesn't stop and that you are still, for the most part, able to do (what) everyone else can," he said.

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"Glee" star Amber Riley also hopes to send a message through her participation.

"You know, I really want young girls that look just like me to feel - just get out there and move and dance," she said.

As for the other celebrity dancers this season, many of them have admitted to feeling first-night jitters.

"Yes, definitely nervous. Very anxious," Riley told Rycroft, who was "Dancing With the Stars: All-Stars" champion in 2012.

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Comedian Bill Engvall said he was more nervous than he thought he would be, while actress Leah Remini admitted feeling "terrified" about her looming dance debut.

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Rounding out the celebrity cast are actress and new mother Elizabeth Berkley Lauren, actors Corbin Bleu and Brant Daugherty, former NFL star Keyshawn Johnson, singer Christina Milian, Nicole "Snooki" Polizzi of "The Jersey Shore," and scientist Bill Nye, who starred in the educational TV show, "Bill Nye the Science Guy."

The 12 celebrities and their professional dance partners tonight will perform either the cha cha, fox trot or contemporary dance.

In a departure from previous seasons, there will be only one, two-hour show per week.

Viewers can vote for their favorite dancing performance each week. Beginning Monday, Sept. 23, judges' scores from that night, along with the public votes from the previous week, will be combined. The couple with the lowest combined total will be eliminated.