The invention provides a composition comprising a mixture of polypeptides, wherein each polypeptide (a) is a copolymer of the amino acids L-glutamic acid, L-alanine, L-tyrosine, and L-lysine, and (b) may be in the form of a pharmaceutically acceptable salt; and wherein in the mixture (i) the polypeptides have an average molecular weight in the range 13,500 to 18,500 daltons, (ii) 13% to 38% of the polypeptides have a diethylamide group instead of a carboxyl group present at one end thereof, and (iii) 68% of the polypeptides have a molecular weight between 7,000 and 41,000 daltons. In an embodiment, the average molecular weight is 16,000 daltons. The invention also provides a method of treating a human subject afflicted with a neurodegenerative disease comprising administering to the human subject a therapeutically effective amount of any of the disclosed compositions so as to thereby treat the human subject.