Disclosed is the characterization and purification of DNA encoding a numerous polypeptides useful for the stimulation of glial cell (particularly, Schwann cell) mitogenesis and treating glial cell tumors. Also disclosed are DNA sequences encoding novel polypeptides which may have use in stimulating glial cell mitogenesis and treating glial cell tumors. Methods for the synthesis, purification and testing of both known and novel polypeptides for their use as both therapeutic and diagnostic aids in the treatment of diseases involving glial cells are also provided. Methods are also provided for the use of these polypeptides for the preparation of antibody probes useful for both diagnostic and therapeutic use in diseases involving glial cells.