On Monday, Steve Ballmer surpassed Bill Gates to become the sixth-richest person in the world, marking the first time the former CEO of Microsoft Corp. has been wealthier than the company’s co-founder.

This milestone was achieved as Microsoft shares soared to a fresh record, resulting in a total gain of 21% this year. The company’s partnership with OpenAI has been a significant factor in its success, benefiting from the artificial intelligence rally that has driven the US stock market higher.

Over 90% of Ballmer’s $157.2 billion net worth is in Microsoft shares, according to the Bloomberg Billionaires Index. In contrast, Gates has diversified his $156.7 billion fortune, with about half his wealth held through Cascade Investment, created with proceeds from Microsoft stock sales and dividends. Additionally, he owns a $21 billion stake in waste-management company Republic Services Inc. through Cascade.

Gates, at 68, has been actively reducing his fortune through philanthropic efforts. Alongside his ex-wife Melinda French Gates and friend Warren Buffett, Gates has channeled billions of dollars from his personal assets into the $75 billion Gates Foundation, one of the largest charitable organizations globally.

Since its inception over two decades ago, Gates and his former wife have donated nearly $60 billion from their personal wealth. Recently, French Gates stepped down as co-chair of the foundation and received $12.5 billion for her own charitable endeavors.

In 2010, Gates, French Gates, and Buffett initiated the Giving Pledge, a movement encouraging the world’s wealthiest individuals to donate a majority of their fortunes during their lifetimes or in their wills. While Ballmer, 68, has not signed the Giving Pledge, he does engage in his philanthropic endeavors, albeit not on the same scale as Gates.

Gates co-founded Microsoft with his friend Paul Allen in 1975 and led the company until 2000 when Ballmer, one of its earliest employees, took over as CEO. Ballmer retired in 2014 and became Microsoft’s largest shareholder that same year. Furthermore, his investment in the NBA’s Los Angeles Clippers in 2014, worth $2 billion initially, is estimated at $4.6 billion today.