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Jan Koum arrived in the United States from Kyiv at sixteen, moving into a small apartment with his mother and grandmother on public assistance while he worked as a grocery-store cleaner and she babysat. He dropped out of San Jose State University to work as an infrastructure engineer at Yahoo, and in 2009 — after Facebook had rejected his own job application — co-founded WhatsApp with former Yahoo colleague Brian Acton. Facebook acquired the messaging app five years later for $19 billion. Koum stepped down from WhatsApp and Facebook's board in 2018 amid disputes over the company's plans for the app, reportedly forfeiting a substantial portion of unvested stock in the process.