Daniel C. Matt offers an English translation of the Zohar that interprets the "mark" or "sign" of Cain mentioned in Genesis 4:15.
Daniel C. Matt, ed. The Zohar, Pritzker Edition. Vol. 1 (Stanford, CA: Stanford University Press, 2004), p. 231
"YHVH put אוֹת (ot), a mark, on Cain (ibid., 15). What is אוֹת (ot), a mark? He place upon him one אוֹת (ot), letter, of the twenty-two letters of Torah to protect him."