Weapon X: Biographical Information On Known Mutants
Jean Grey came to know death (literally) at quite a young age. And that death would forever change her entire life.
When she was just ten years old, Jean saw her best friend Annie Richardson get struck and killed by an automobile. Touching her dying friend, Jean's emotions awakened her latent telepathic powers and she experienced Annie's dying thoughts as if they were her own.
Jean was unable to cope, and also began exhibiting telekinectic abilities. When her powers manifested, her parents became extremely fearful. Jean was having headaches and made all of the cars and mailboxes in her neighborhood fly into the air.
Professor Xavier promised Jean's parents that he would help her to control her mutant powers, and he did for years until that fateful day when Jean died and was later "reborn" with evolved mutant abilities.
Jean's powers increased to the point where she could kill humans and mutants alike with a simple thought. Those sometimes uncontrollable thoughts killed her fiance Scott Summers and Professor Xavier, who had put mental blocks on Jean when she was a child, blocking her additional abilities.
Jean was later killed on Alcatraz by Logan aka 'Wolverine'.