were any of them assasinated?
Yes, John F. Kennedy.
Twenty-fifth Amendment to the United States Constitution
 The assassination of John F. Kennedy in 1963 demonstrated to policymakers the need for a clear procedure for determining presidential disability, especially in the context of the Cold War. 
what is the 25th amendment to the constitution say It deals with issues related to presidential succession and disability. how many sections does it have? UNANSWERABLE this is the constitution of the united states of america, correct? Yes when was it drafted? UNANSWERABLE who drafted it? UNANSWERABLE who was the first president? UNANSWERABLE
wiki:13393582
Twenty-fifth Amendment to the United States Constitution [SEP] Kennedy assassination
By the 1960s, medical advances had made increasingly plausible the scenario of an injured or ill president living a long time while incapacitated. The assassination of John F. Kennedy in 1963 demonstrated to policymakers the need for a clear procedure for determining presidential disability, especially in the context of the Cold War. The new president, Lyndon B. Johnson, had once suffered a heart attack andwith the office of vice president to remain vacant until the next term began on January 20, 1965the next two people in the line of succession were the 71-year-old speaker of the House John McCormack and the 86-year-old Senate president pro tempore Carl Hayden.