Dr. Berthelsen has served as a director of Ionis since May 2002. Since 1980, he has practiced Internal Medicine with the Kelsey Seybold Clinic, a 400-physician medical group based in the Texas Medical Center in Houston. Dr. Berthelsen has served in various senior leadership positions at Kelsey Seybold, including chairman of the Department of Internal Medicine, medical director and managing director. He served as chairman of their Board of Directors from October 2001 through April 2016. He has served as a clinical professor of Medicine at Baylor College of Medicine and the University of Texas Health Science Center of Houston. Dr. Berthelsen served on the board of the Texas Academy of Internal Medicine in the past and the Caremark National Pharmacy and Therapeutics Committee from 1999 through 2005.