Microsoft ( MSFT) Chairman Bill Gates is giving up his directorship with Icos ( ICOS), a drug developer, to spend more time working on the board of Warren Buffett's Berkshire Hathaway ( BRKa), according to a filing with the Securities and Exchange Commission. Icos is best known for developing Cialis, a popular drug used to treat erectile dysfunction. Gates said that Buffett, a longtime friend, asked him to join the Hathaway board shortly before Christmas. "Given my friendship with Warren and my interest in helping out in any way he ever asks, Berkshire is now my highest priority for board membership," Gates wrote in a letter to Icos Chairman Paul Clark. Gates said he intends to remain a long-term investor in Icos.