The present invention relates to a purified retinal pigmented epithelium derived neurotrophic factor composition and a method for purifying such a retinal pigmented epithelium neurotrophic factor. The present invention also relates to a recombinant DNA molecule comprising a gene encoding a retinal pigmented epithelium derived neurotrophic factor having the DNA sequence or the amino acid sequence in SEQ ID NO:1 and to an organism transformed with the recombinant DNA molecule.In addition, the present invention relates to a method of treating tumors, ocular diseases, nerve injuries, and conditions resulting from the activity of serine proteases, which comprises administering PEDF.