Mentor Graphics extended a "shareholder rights plan" to block hostile takeovers.
Such plans, better known as "poison pills," give existing shareholders the right to buy more stock at a discount if someone else's holdings exceed a set percentage of the company's stock.
Wilsonville-based Mentor adopted its poison pill in 2010 when activist investor Carl Icahn was accumulating the company's stock. He later waged an unsuccessful campaign to replace the company's management or have it sold, though he did win a proxy fight to put some of his representatives on Mentor's board.
Read more at OregonLive.com.