There are four possibilities that are mentioned in Midrashic sources as to what the Mark of Cain was. According to the Midrash Bereshit Rabbah, 22:28 with Tifferet Tziyon, Cain became a leper, another option is that God gave him a dog to guard him, ibid., a horn grew out of his forehead, ibid., and the Midrash Aggadah 4:15 states that God engraved a letter of His Divine Name on Cain's forehead to protect him.
