Warren Buffett holds an estimated $56.5 billion worth of Berkshire Hathaway stock — the most valuable stock holdings of any active chief executive officer. Oracle CEO Larry Ellison and Amazon.com CEO Jeff Bezos have the next most valuable company stock holdings.

As many as 27 CEOs hold at least $1 billion worth of their companies’ stock, according to the most recent stock prices and ownership information. To determine the 15 CEOs with the most valuable stock ownership by market value, 24/7 Wall St. reviewed company proxy statements and recent share prices for companies with a market cap greater than $5 billion. These are the CEOs whose companies made them rich.

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Many of the nation’s wealthiest CEOs founded the companies they run. Both Harold Hamm and Ralph Lauren began to build their current businesses in 1967, with Hamm starting out searching for oil and Lauren selling ties. In all, 12 of the 15 wealthiest CEOs are also their company’s founders. Some of these executives made all, or effectively all, of their money after founding their companies. Michael Dell and Mark Zuckerberg famously founded their businesses while in college.

Only a few of these executives made their money before their current position. Elon Musk, the current CEO of Tesla, made his first fortune from the sale of PayPal. Sheldon Adelson, founder of resort and casino giant Las Vegas Sands, first owned a successful computer trade show, called COMDEX. Adelson sold COMDEX after founding Las Vegas Sands for $800 million.

Several of these executives continue to be actively involved in multiple companies. Rupert Murdoch recently split his longtime business into two companies, Twenty-First Century Fox and News Corporation. While he is only the CEO of the former, he remains the chairman of both.

Among the three wealthiest CEOs who are not the founders, two are closely connected to the company’s founder or founding group. Steve Ballmer joined Microsoft early in its history as its first business manager and received 8% of the company’s stock on the day it incorporated. John Hess, while not the founder of the Hess Corporation, is the son of founder Leon Hess and has been the CEO of the company for more than 28 years. However, Hess was removed as the company’s chairman in favor of an independent, non-executive chairman.

Many companies with extremely wealthy CEOs are in the technology sector. This includes individuals such as Ballmer, Michael Dell and Larry Ellison of Oracle, who played major roles in developing and marketing much of the technology used today. Later, Jeff Bezos, Larry Page and Mark Zuckerberg founded companies that provide online services to hundreds of millions, or even billions, of people worldwide.

Another well-represented industry among the wealthy CEOs is the energy industry, which includes three of the nation’s wealthiest CEOs. Among these are Harold Hamm, whose company, Continental Resources, is one of the nation’s leading energy exploration and production companies. Kinder-Morgan, led by CEO Richard Kinder, has focused mostly on developing pipelines and oil terminals.

To determine the 15 CEOs with the most valuable company holdings, 24/7 Wall St. reviewed figures produced by Capital IQ. Each CEO’s total number of shares beneficially owned are based on the most recently available proxy statement. Additionally, prices used in determining total market value of their holdings are accurate as of September 8. Because stock prices fluctuate, and executives may buy or sell shares in their companies, these numbers likely will have changed slightly.

These are the CEOs whose companies made them rich.