Genesis of a novel (East of Eden)

When John Steinbeck was writing his first novel, East of Eden, he kept a notebook of letters to his editor. From these it seems that well into the second part of the book, he had intended to call it 'Cain's Sign'. Doesn't quite have the same ring!
But why? Here's what the man says himself:
"I thought about the book a great deal yesterday - what it is about and what its title should be. It is not local. It is not primarily about the Salinas valley nor local people. Therefore it should have a general title. Now - its framework roots from that powerful, profound and perplexing story in Genesis of Cain and Abel. There is much of it I don't understand. Furthermore it is very short, but this story with its implications has made a deeper mark in people than any other save possibly the story of the Tree of Life and original sin. ... With this in mind I went back to Genesis ...
The punishment of Cain is a strange and perplexing one. Out of Eve's sin came love and death. Cain invented murder and he is punished by life and protection. The mark put on him is not placed there to punish him but to protect him. Have you ever thought of that? And this is the best known mark in the world. so I suggest as a title for my book Cain Sign. ... What do you think of it?"