Leoni is serving as honorary chairwoman for the seventh consecutive year. Along with Messing, the list of honorary hosts for Saturday's hike includes supermodel Cindy Crawford, Jeanne Tripplehorn ("Waterworld"), fitness expert Kathy Smith, Kim Raver ("24") and Richard Roundtree ("Shaft").
Leoni told United Press International there are a lot of ways to raise money and awareness for breast cancer, but she likes this one best.
"The hike is symbolic for us as women, as mothers, as husbands, as brothers, fighting back actively, climbing a mountain," she said. "I prefer this to the sit-down dinners with Chicken Kiev."
Brenda Himelfarb, a breast-cancer survivor, is in her fourth year as chairwoman of the event, which raises funds to benefit Expedition Inspiration Fund for Breast Cancer Research, and the Revlon/UCLA and USC/Norris Lee Breast Centers, through UCLA's Jonsson and the USC/Norris Comprehensive Cancer Centers.
A freelance writer and public-relations consultant, Himelfarb helped found the Vail Breast Cancer Awareness in Vail, Colo., before moving to California. One of that organization's biggest fundraising activities is billed as the biggest snowshoe race in the world.
"Part of staying healthy is being in shape," said Himelfarb.
After she was diagnosed with LCIS -- lobular carcinoma in situ, which puts one at high risk for breast cancer -- Himelfarb entered the High Risk Program at the Revlon/UCLA center. After a malignant lump was found, she received radiation treatment and took tamoxifen for five years -- and is living cancer-free.
(Researchers at the University of North Carolina at Chapel Hill recently reported that, although tamoxifen citrate can keep breast cancer from occurring in women, it can also cause possibly harmful side effects -- including blood clots and stroke -- that would rule out its use by some 90 percent of women who might otherwise benefit from taking it.)
Himelfarb assists Sherry Goldman, director of the High Risk Program at the Revlon/UCLA center, in teaching a class for patients who are about to undergo breast surgery. She has written -- and plans to publish -- a text, "Breast Cancer 101," which she said offers useful information in language people without medical backgrounds can understand.
"When doctors write about this, everything is in 'doctor,' no matter how they try," she said.
Doctors, it turns out, are among the biggest boosters of Expedition Inspiration Take-A-Hike.
Dr. Michael Press, a surgical pathologist at the Los Angeles County Women's and Children's Hospital and the USC Medical Center, is a regular participant in the annual hike. He said raising awareness about breast cancer is vital to helping prevent and treat it.
"I think the message really begins with the person, that they have to be aware of how this disease potentially impacts them," he said. "Have they had checkups? Are they exercising regularly? Are there people in their lives that they know who are affected, who they can help in some way?"
Press is best known for his work in characterizing molecular genetic alterations of breast cancer and in evaluating methods used to identify genetic alterations in other human cancers.
He said cancer research has recently made progress in targeted therapy but is still not within reach of what the public might think of as a cure for cancer.
"We tend to see breast cancer as if it were a single disease. I think it should be viewed as many different diseases," he said. "The molecular alterations are quite variable. It will be quite some time before we'll be able to treat all of them effectively."
Expedition Inspiration (expeditioninspiration.org) had its beginnings in the early '90s, when Laura Evans -- struggling with breast cancer -- decided to assemble a team of breast-cancer survivors to climb a mountain to call attention to breast cancer and raise funds for research. In January 1995 Evans and mountain guide Peter Whittaker led a team of 17 breast-cancer survivors to the top of Argentina's Mount Aconcagua -- the highest peak in the Western Hemisphere, at 22,841 feet.
Evans, who died of brain cancer in 1999, wrote about the climb in her book, "The Climb of My Life." Expedition Inspiration holds an annual cancer symposium in her name, Himelfarb said.
The Take-A-Hike challenge is considerably less rigorous than the Mt. Aconcagua trek.
The celebrities on hand will lead hikes geared to all ages and fitness levels, and Leoni said they don't just put in an appearance for a few minutes and then leave -- they hike the whole way. She also said they don't present themselves as experts on the subject.
"I don't speak with any more authority than anybody else who is touched by this disease personally or otherwise," she said, "just as somebody who is concerned about the growing devastation of this disease, and hopeful that we can find a cure -- or that preventative measures can become so advanced that we can diagnose this disease before it causes anyone any pain. Just like any other mother, I hope that for my children."
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Notable deaths of 2014 [PHOTOS]