When the polypeptide is an agent that inhibits the expression or activity of netrin or of netrin signaling, exemplary biological activities retained by a modified polypeptide for use in the methods of the present invention include one or more of the following: (i) inhibits attraction of axons; (ii) inhibits angiogenesis, (iii) inhibits cell migration, (iv) inhibits cell adhesion, (v) inhibits cell