Microsoft Chairman Bill Gates has resigned from the board of directors of Icos after serving 15 years. In a letter to Icos Chief Executive Paul Clark, Gates said he is leaving...

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Microsoft Chairman Bill Gates has resigned from the board of directors of Icos after serving 15 years.

In a letter to Icos Chief Executive Paul Clark, Gates said he is leaving the Bothell biotech company’s board Feb. 9 because he has always tried to limit his involvement to only one outside public board.

Last month, Gates joined the board of Berkshire Hathaway, the company run by his friend, billionaire investor Warren Buffett.

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Icos was launched in 1990 by a star-studded cast of scientists and business leaders. It aimed to develop treatments for chronic inflammatory diseases and eventually went on to develop Cialis, the drug for erectile dysfunction.

In his letter, Gates wrote: “I plan to be a long-term shareholder of the company.”

“Icos is a great company with an exciting future ahead of it,” he wrote. “With any luck, the drugs in the pipeline will not only generate great returns for shareholders but also have significant impact in improving health.”

As of September, Gates was the third-largest shareholder in Icos, with 5.7 million shares, or about 9 percent of the company’s shares.

Historically the largest stockholder, Gates was surpassed last year by Primecap Management and Wellington Management, according to Bloomberg News data.

Icos spokeswoman Lacy Fitzpatrick said Gates has not sold any Icos shares to date.