Michael Maher offers an English translation of Targum Pseudo-Jonathan that includes a paraphrase of Genesis 4:15 discussing the "mark" or "sign" of Cain.
Michael Maher, ed. Targum Pseudo-Jonathan, Genesis. Vol. 1b. Aramaic Bible (Collegeville, MN: Liturgical Press, 1992), 34
Then the Lord traced on Cain's face a letter of the great and glorious Name, so that anyone who would find him, upon seeing it on him, would not kill him.