WILSONVILLE, OR -- Mentor Graphics' board has adopted a Shareholder Rights Plan, granting each of its shareholders the opportunity to purchase one incentive stock purchase for each share of common stock currently held.

The option, which goes into effect if a person, acting alone or as part of a group, accumulates 15% or more of Mentor stock (or announces an offer to do so). The decision comes on the heel of an announcement that noted corporate raider Carl Icahn has accumulated 9.5% of Mentor's stock. The option gives current shareholders the right to purchase an additional share at 50% of the price of the current price.

Unless redeemed earlier by the company, the rights expire Dec. 31, 2011.

The move "enables the board to protect the company and to allow all of the shareholders to realize the long-term value of their investment," the firm said in a press release.

Ed.: Comment below, or see Mike Buetow's blog on this topic.