Abraham Levene translates a commentary on Genesis 4:15 from early Syrian Christians mentioning the "mark" or "sign" of Cain.
Abraham Levene, The Early Syrian Fathers on Genesis from a Syriac MS. on the Pentateuch in the Mingana Collection (London: Taylor’s Foreign Press, 1951), 80
It says "a sign," i.e., He appointed it, that he [Cain] should not be killed, just as "Thou hast given a sign to them that fear Thee," i.e., "Thou hast guarded us as with a sign."