NEW YORK — ABC, CBS and NBC have set aside a prime-time hour on the Friday after Labor Day to simulcast a fundraiser for cancer research.

"Stand Up to Cancer" on Sept. 5 will feature musical performances and celebrity appeals, although no guests were immediately announced last week.

The rare simultaneous broadcast is reminiscent of a benefit for the Sept. 11 terrorist attacks that was shown on many networks weeks later.

Fox, which just completed the TV season as the nation's most-watched network for the first time, was asked to participate but declined. Fox said it was concentrating on its "Idol Gives Back" initiative but would run public service announcements promoting the cancer special.

"For people struggling with this disease, or those who will be diagnosed, scientific breakthroughs can be a matter of life or death — literally," said Couric, whose husband and sister both died of cancer. "We want everyone to know that they can make a difference in this fight."

Organizers will work with the American Association for Cancer Research to distribute the money. They are looking for researchers who are close to breakthroughs on how new findings on cancer can be translated into treatments, said Laura Ziskin, the event's producer.

An estimated 20 percent of the money raised will be directed to "out-of-the-box" research efforts that normally have trouble getting funds, said Ziskin, who has breast cancer.

"It's a populist cancer movement," she said.

Networks took the unique step of having their three evening newscasters — CBS' Couric, ABC's Charles Gibson and NBC's Brian Williams — appear together on the three morning shows May 28 to make the announcement.

But NBC blew its cover the day before when Matt Lauer announced that Couric, his former partner on "Today," would be back there the next day for a major announcement. Couric, the former queen of morning television at "Today," hadn't been back on that show's set since her March 31, 2006, farewell episode.

She's since moved on to anchor the "CBS Evening News."

Ziskin said she was inspired by Al Gore's movie, "An Inconvenient Truth," and how it affected the national conversation on global warming. She hoped to accomplish the same thing with an effort about cancer.

Couric has been an active fundraiser for cancer treatment and research since her husband, Jay Monahan, died of colon cancer in 1998. She underwent an on-air colonoscopy that encouraged so many people to do the same that it was called "The Couric Effect."

The American Cancer Society predicts 1.4 million new cancer cases will be diagnosed this year.