The present invention relates to RSPO-binding agents and methods of using the agents for treating diseases such as cancer. The present invention provides antibodies that specifically bind human RSPO proteins and modulate ß-catenin activity. The present invention further provides methods of using agents that modulate the activity of RSPO proteins, such as antibodies that specifically bind RSPO1, RSPO2, and/or RSPO3 and inhibit tumor growth. Also described are methods of treating cancer comprising administering a therapeutically effect amount of an agent or antibody of the present invention to a patient having a tumor or cancer.
