John C. Martin, Ph.D., chairman and chief executive officer of Gilead Sciences, Inc. will step down, effective March 10, 2016, and will assume the role of executive chairman of the company. John F. Milligan, Ph.D., currently president and chief operating officer, will be promoted to chief executive officer and will serve on the board of directors.   Dr. Martin joined the company in 1990 and was appointed chief executive officer in 1996. Dr. Milligan also joined Gilead in 1990 and was appointed...