In chapter 4 of Genesis in the Catholic Bible, what is the mark of Cain?

Cain's whimpering complaint reveals remorse for the consequences of his sin rather than for its guilt. But even then the Lord allayed the fugitive's fears for his life by putting a protective mark on Cain and a curse on anyone who killed him. Cain went out from the presence of the Lord, the saddest of all departures.