Therefore = not so (with Septuagint) This is emphasized by the Hebrew accent (Pasha).
upon = set a sign for Cain, i.e. gave him a pledge; same word as Gen_1:14; Gen_9:12, Gen_9:13, Gen_9:17 (token). See also Exo_4:8, Exo_4:9, Exo_4:17, Exo_4:28, Exo_4:30; Exo_12:13. Eze_20:12.