The present invention provides isolated monkey Cathepsin S proteins and isolated nucleic acid molecules such as cDNA molecules that encode these monkey Cathepsin S proteins. The present invention specifically provides, for example, isolated monkey Cathepsin S proteins and encoding nucleic acid molecules, methods of producing the monkey Cathepsin S proteins, methods of identifying and screening modulators of the monkey Cathepsin S proteins, and methods of using these modulators to treat a human disease or disorders associated with an orthologous human Cathepsin S protein.