Yaakov from Abbotsford, British Columbia, Canada wrote:
Is there any reference - Talmudic or otherwise - that tells us what the mark was that Hashem placed upon Cain to protect him (Genesis 4:15)?
Here are a number of opinions from Midrashic sources:
- Cain became a leper so people would avoid him.
- G-d gave him a dog to guard him.
- A horn grew out of his forehead for protection.
- G-d engraved a letter of His Name on Cain's forehead. This would remind people that G-d commanded not to kill him.
- Bereishet Rabbah 22:28, Tifferet Tziyon
- Midrash Aggadah 4:15