Yaakov from Abbotsford, British Columbia, Canada wrote:

Dear Rabbi,

Is there any reference - Talmudic or otherwise - that tells us what the mark was that Hashem placed upon Cain to protect him (Genesis 4:15)?

Dear Yaakov,

Here are a number of opinions from Midrashic sources:

• Cain became a leper so people would avoid him.
• G-d gave him a dog to guard him.
• A horn grew out of his forehead for protection.
• G-d engraved a letter of His Name on Cain's forehead. This would remind people that G-d commanded not to kill him.

Sources:

• Bereishet Rabbah 22:28, Tifferet Tziyon
• Midrash Aggadah 4:15