Samuel A. Berman provides a Hebrew transcription of the Midrash Tanchuma discussion of Genesis 4:15, which mentions Cain's "mark" or "sign."
Samuel A. Berman, ed., Midrash Tanhuma-Yelammedenu: An English Translation of Genesis and Exodus from the Printed Version of Tanhuma-Yelammedenu with an Introduction, Notes, and Indexes (Hoboken, NJ: KTAV Publishing, 1996), Bereshit 10, 31
כְּשֵׁם שֶׁלִּמֵּד שַׁבָּת זְכוּת עַל אָדָם הָרִאשׁוֹן, כָּךְ לִמֵּד עַל קַיִן. וְיֵשׁ אוֹמְרִים: קֶרֶן קָבַע בְּמִצְחוֹ.
English translation (Berman): There are some who say that the word "Sabbath" was placed as a sign upon his countenance, as it is said: My Sabbaths, for it is the sign between Me and you, throughout your generations (Exod. 3:13), and that just as the Sabbath pleaded in behalf of Adam, it pleaded in behalf of Cain. Others, however, insist that He fastened a horn upon his forehead.