Someone mentioned "missing evidence". From what I've read, this "evidence", physical tissue samples taken from JFK, was turned over to Robert Kennedy. The Kennedy family did not want this material to be on display in museums in the future and they had fought to keep items such as the autopsy photos from being made public.   Again, from what I've read, the trail of what happened to this material after it was given to RFK is unknown. It is assumed that he had it destroyed.