Either way, the search for a new Biogen CEO, which the company said is already underway, will be an interesting process to watch. Now that Mullen is essentially a lame duck, it's reasonable to expect the company's board -- including Icahn's proxies -- to take a more active management role.
Naturally, speculation will increase that Biogen could be placed back on the sale block.
It might be lost amidst all the palace intrigue, but Mullen did manage to bring home a 12% return for shareholders in 2009 -- making Biogen the only big-cap biotech stock to close the year in the green.
Still, Biogen does face challenges ahead with the increase in the number of cases of the serious brain infection known as PML tied to use of its multiple sclerosis drug Tysabri. The company has also experienced setbacks recently with its late-stage drug pipeline.Biogen shares closed Monday at $53.64, ahead of the announcement of Mullen's retirement.