Subway was started in 1965 when 17-year-old Fred DeLuca asked Dr. Peter Buck, a nuclear physicist who was a family friend, for advice on how to pay his college tuition fees. Buck invested $1,000 to help DeLuca open a sandwich shop in Bridgeport, Conn.—starting a business that now spans more than 100 countries.

Having never gone public, Subway remained in family hands—but much of the business is now set to be held by a private charity.

The Peter and Carmen Lucia Buck Foundation—founded by Buck and his late wife in 1999—announced on Monday that the late cofounder had bequeathed his 50% stake in Subway to the charity.

“Dr. Buck planned this generous bequest over a decade ago as the final piece of his philanthropic plan to build PCLB into an institution designed to promote the best qualities of human nature,” the organization said.

The charity provides grants and other assistance including guidance and professional connections in a bid to give “motivated people the tools they need to help themselves.” Its interests include education, medicine, and journalism.

Spokespeople for the Peter and Carmen Lucia Buck Foundation were not available when contacted by Fortune.

Members of the Buck family sit on the charity’s board of directors. Forbes—citing a copy of Buck’s will—reported that the late Dr. Buck’s sons Christopher and William, both of whom are board members, and Ben Benoit, its CFO, were executors of his estate.

When Buck passed away, the publication estimated his net worth was $1.7 billion.

Earlier this month, the Wall Street Journal reported that the sandwich giant was exploring a sale that could value the company at $10 billion, meaning Buck’s gift to his foundation could be worth as much as $5 billion.

The deal, if it went ahead, could have made the heirs to Subway some of the wealthiest people in America.

Representatives for the sandwich chain told Fortune on Wednesday that the company would not comment on ownership structure and business plans.

The Peter and Carmen Lucia Buck Foundation said in its announcement on Monday that the Buck and DeLuca families “remain close.” According to Insider, members of both families are members of Subway’s board of directors.

“This gift will allow the foundation to greatly expand its philanthropic endeavors and impact many more lives, especially our work to create educational opportunities for all students, work Dr. Buck cared so deeply about,” Carrie Schindele, executive director of the charity, said in a statement.

In recent years, Subway has appeared to face financial and operational struggles, reportedly closing 10% of its stores in 2020 and laying off hundreds of its corporate workers.

In 2021, a “concerned group of Subway franchisees” wrote to co-owner Elisabeth DeLuca to ask her to fix what they called “nightmare” franchise terms.

Last year, however, the company said it was seeing record sales thanks to a transformation strategy that included a menu overhaul, store remodels, and upgrades to its app.