Bill Gates unloaded his personal stake in Seattle Genetics, gifting more than 3.5 million shares to the Bill & Melinda Gates Foundation on Valentine's Day, according to a regulatory filing Friday.

Shares in the Bothell biotechnology company closed at $4.79 that day, making the gift to the global charity worth $16.9 million.

Gates bought into the company in 2001. His stake represented 8.3 percent of the company's outstanding shares. Stock held by the foundation is deemed beneficially owned by Bill and Melinda Gates its co-trustees.