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Jim Stowers Jr., the billionaire founder of one of the nation's leading investment management firms who gave away most of his fortune to fight disease, has died. He was 90.

The Kansas City, Mo., philanthropist, whose tenacity in business was just as fierce as in his fight for stem-cell research, died Monday after a period of declining health, according to a news release issued by his namesake research firm and the investment firm he founded, American Century Investments.

Stowers was a struggling mutual fund salesman in 1958 when he founded Twentieth Century Investors Inc. with only two mutual funds and $107,000 in assets. That company grew into American Century Investments, one of the nation's leading investment management firms that currently manages about $141 billion in assets.

In 2000, Stowers and his wife, Virginia, who both successfully fought cancer, promised more than $1 billion of their fortune to create the Stowers Institute for Medical Research in Kansas City. The endowment eventually grew to $2 billion.

"I've always said that if I make other people successful, they'll make me successful," he told The Associated Press in 2009. "We wanted to give something that was more valuable than money to the millions of people who made our success possible."

The gleaming institute has attracted world-renowned researchers to Kansas City and prompted civic leaders in the region to enhance collaboration between area research groups, health care organizations, universities and business interests.

"For Jim, creating new knowledge was the most powerful contribution he could offer mankind," said Richard W. Brown, chairman of the board of American Century Investments and the Stowers Institute for Medical Research. "Throughout his whole life, whether as businessman or philanthropist, he thought about making things better for other people."

Stowers and his wife said they wanted the institute to focus on basic research into how genes work, to determine ways to alter genes to fight such ailments as cancer, heart disease and Alzheimer's disease. But they faced skepticism from well-known U.S. research institutions about whether the Kansas City area could be a major player in biomedical research and life sciences.

By 2013, the institute had about 370 scientists, research associates, technicians and support staff.

"We were told that what we wanted to do couldn't be done here," Stowers told the AP in the 2009 interview. "We had a whole bunch of labs tell us that they could do a better job than us.

"You know, I had the same statements made to me when I started American Century. I just said, `I can prove it, I can do this."'

Stowers was born in Kansas City. He earned bachelor's degrees in art and medicine from the University of Missouri, and was an Air Force fighter pilot during World War II.

He and wife, Virginia, with whom he had four children, said they were motivated to help find cures by their bouts with cancer. Stowers was diagnosed with prostate cancer in 1987, and his wife had surgery for breast cancer in 1993.

Stowers' efforts pushing stem-cell research sparked controversy in 2006, when state voters narrowly approved a constitutional amendment guaranteeing that any federally allowed stem-cell research and treatment could occur in Missouri.

The Stowerses donated most of the $30 million spent by The Coalition for Lifesaving Cures, an organization formed to support the amendment, saying stem-cell research was important to the institute's efforts to fight disease.

The amendment was only narrowly approved, after a bitter political fight in which opponents claimed the use of embryonic stem cells for research required the destruction of a human embryo. Supporters contend the research does not end a life and had the potential to cure many debilitating diseases.