Neurotrophic factors which act on motor neurons may provide an efficient treatment for amyotrophic lateral sclerosis. In order to overcome the difficulty of administering proteins into the central nervous system, it is possible to use a gene therapy approach, i.e. the polymer encapsulation of cells which have been genetically engineered to release one of these neurotrophic factors. The polymer-encapsulated cells can be implanted subcutaneously or even intrathecally and permit continuous, slow release of proteins. There is no rejection of the cells due to their isolation by a semi-permeable membrane, no risk of tumor formation and the polymer device can be removed in the event of a problem.