Yahoo CEO Marissa Mayer is losing a precious security blanket now that she is spinning off the Internet company's prized stake in China's Alibaba Group Holding Ltd.

The breakup announced on Tuesday will transfer ownership of 384 million shares of Alibaba stock, currently worth $39 billion, into a new entity called SpinCo. Those holdings, part of an astute investment made nearly a decade ago, represent the main reason that Yahoo's stock has more than tripled since Mayer became CEO two-and-half years ago.

Investors viewed Yahoo as another way to own a piece of Alibaba, a rapidly growing e-commerce company that is expected to become even more successful during the next decade as more of China's population gains online access through smartphones and other devices.