Steve Ballmer passed Bill Gates on Monday to become the sixth-richest person in the world, the first time the former chief executive officer of Microsoft Corp. has been wealthier than the company’s co-founder.

The move came as Microsoft shares rose to a fresh record, bringing their total gain this year to 21%. The company, through its partnership with OpenAI, has been one of the biggest beneficiaries of the artificial intelligence rally that has powered the US stock market higher.

More than 90% of Ballmer’s $157.2 billion net worth is in Microsoft shares, according to the Bloomberg Billionaires Index. Gates, meanwhile, has diversified his $156.7 billion fortune: About half his wealth is held through Cascade Investment, which was created with the proceeds of Microsoft stock sales and dividends. He also owns a $21 billion stake in waste-management company Republic Services Inc. through Cascade.