KANSAS CITY, Mo. — 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, as of 2013, was managing about $141 billion. 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. 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.