Jacob Neusner provides an English translation of Genesis Rabbah 22:12 on Genesis 4:15, which mentions the "mark" or "sign" of Cain.
Jacob Neusner, ed. Genesis Rabbah, vol. 1, ed. Jacob Neusner (Atlanta, GA: Scholars Press, 1985), p. 253
2. A. "And the Lord put a mark [on Cain, lest any who came upon him should kill him]" (Gen. 4:15):
B. R. Judah said, "He made the orb of the sun shine for him."
C. Said to him R. Nehemiah, "Do you think that for that wicked man God would make the orb of the sun shine? Rather he made saraat [leprosy] break out on him. . ."
3. A. Rab said, "He gave him a dog." [That is the sign of Cain.]
B. Abba Yose said, "He made a horn grow on him."