In the beginning, Steinbeck used 'The Salinas Valley' as the working title of East of Eden when he began to write it (simply 'S.V.' in his notes and letters to his friend and editor, Pascal Covici). His later idea of calling it 'Cain's Sign' was short-lived, but the reason he wanted a Biblical reference to Genesis in the title was fundamental to the story.
So the name of the book became a subject of correspondence with his editor "... you must know that I am stating my thesis and laying it out. And I am glad that I can use the oldest story in the world to be the design of the newest story for me".
Steinbeck said that this Genesis story was "the basis of all human neurosis - and if you take the fall along with it, you have the total of all the psychic troubles that can happen to a human".
When he wrote down the 16 verses of the story of Cain and Abel in his own hand, the title came to him along with a clearer vision of what he wanted to say.
"The story changes with flashing lights when you write it down. And I think I have a title at last, a beautiful title, EAST OF EDEN. And read the 16th verse to find it. And the Salinas Valley is surely East of Eden."
The next day the question was still burning on Steinbeck's mind:
"I have finally I think found the key to the story. The only one that has ever satisfied me ... my analysis which is going in [to the draft book] today should interest you. It should interest scholars and it should interest psychiatrists.
... The reader I want will find the whole book illuminated by the discussion: just as I am.
And if this were just a discussion of Biblical lore, I would throw it out but it is not. It is using the Biblical story as a measure of ourselves."
So East of Eden as a title was born, and the central metaphor of the story blossomed with a new life.