In Foldit, CPK is a coloring option, found on the View menu. It's similar to the EnzDes coloring option. See the amino acid gallery for the appearance of each amino acid with EnzDes coloring. The name CPK is taken from the CPK coloring convention used for ball-and-stick molecular models. "CPK", refers to Robert Corey, Linus Pauling, and Walter Koltun, who developed the system. CPK coloring only affects the sidechain. The backbone and the carbon atoms in the sidechain are grey. (EnzDes uses green instead of grey.)