Genesis 4:15

“Not so!” replied the Lord. “If anyone slays Cain, then Cain will be avenged sevenfold.” And the Lord placed a mark on Cain, so that no one who found him would kill him.

Note LXX, Vulgate, and Syriac; Hebrew “Very well!” or “Therefore:”

BSB · Berean Standard Bible
And the Lord said unto him, Therefore whosoever slayeth Cain, vengeance shall be taken on him sevenfold. And the Lord set a mark upon Cain, lest any finding him should kill him.
KJV · King James Version
The Lord said to him, “Therefore whoever slays Cain, vengeance will be taken on him sevenfold.” The Lord appointed a sign for Cain, so that anyone finding him would not strike him.
WEB · World English Bible
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.
ASV · American Standard Version

Related passages

From the Treasury of Scripture Knowledge, grouped by the word they explain.

Therefore
1 Kings 16:7; Psalm 59:11; Hosea 1:4; Matthew 26:52
sevenfold
Genesis 4:24; Leviticus 26:18, 21, 24, 28; Psalm 79:12; Proverbs 6:31
Genesis 4:15
set a mark, etc. Or, rather, "gave a sign or token to Cain, that those who found him should not kill him."
Ezekiel 9:4, 6; Revelation 14:9, 11