Genesis 4:15

"And Jehovah said unto him, Therefore, whosoever slayeth Cain, vengeance shall be taken on him sevenfold. And Jehovah appointed a sign for Cain, lest any finding him should smite him."

What was the sign or "mark" that God placed upon Cain? As far as we are able to find out, there is utterly no way whatever to determine this. Ancient traditions about it are worthless, and certainly the notion that it "was some kind of tatoo" (Neil) is equally so. Some have supposed that it was something in the visage or appearance of Cain, but there is nothing substantial that we may find in any such opinions. Of interest is the supposition by some that it was a certain kind of dog that God gave to accompany him, but there's no dependability in that either. Of greater interest is the fact that God did not punish Cain with death immediately. But this was not done, in all probability, because it was God's purpose to allow those generations immediately after Adam to run their course in headlong wickedness which would issue ultimately in a new beginning for humanity, following the Flood. Of significance too is the thought that the mercy of God for Cain was still available had he been willing to seek it.