The amount of active Robo expressed on a cell is modified by modulating the effective amount of a Comm polypeptide in contact with the cell, whereby the amount of expressed active Robo is modulated inversely with the modulation of the effective amount of the Comm polypeptide in contact with the cell. In a particular embodiment, the Comm polypeptide is provided to the cell by exogenously in a pharmaceutically acceptable composition. In another aspect, the invention provides methods of screening for agents which modulate Robo-Comm interactions. These methods generally involve forming a mixture of a Robo-expressing cell, a Comm polypeptide and a candidate agent, and determining the effect of the agent on the amount of Robo expressed by the cell.