Yahoo! headquarters in Sunnyvale, Calif.

Yahoo announced it is looking at cashing in its stake in Yahoo Japan, which could be worth almost $9 billion before tax, news that boosted Yahoo's stock price.

The Internet company has hired advisers to help it evaluate options for the stake, Chief Executive Officer Marissa Mayer told investors on a conference call on Tuesday. It will not be included in the planned spin-off of its stake in China's Alibaba Group Holding, she said.