J. W. Etheridge offers an English translation of the Targum Onkelos that paraphrases Genesis 4:15 about the "mark" or "sign" of Cain.
J. W. Etheridge, ed. The Targums of Onkelos and Jonathan Ben Uzziel on the Pentateuch, with the Fragments of the Jerusalem Targum from the Chaldee. 2 vols. (London, England: Longman, Green, Longman, and Roberts, 1862, 1865), 43
And the Lord set unto Kain a sign, lest any one who found him should kill him.