Rabbi Y. Herczeg translates Rashi commentary into English, with its interpretation of Cain's "mark" or "sign" as mentioned in Genesis 4:15.
Rashi, The Torah with Rashi's Commentary Translated, Annotated, and Elucidated. Vol. 1: Beresheis/Genesis. Translated by Rabbi Yisrael Isser Zvi Herczeg. ArtScroll Series, Sapirstein Edition (Brooklyn, NY: Mesorah Publications, 1995), 46
[God] etched for [Cain] . . . a letter of His name . . . on His forehead.