GDNF and amyotrophic lateral sclerosis: Despite raising levels of exogenous IGF-1 in the spinal cord, intramuscular or intrathecal administration of IGF1-TTC had no significant effect on disease progression or survival of ALS model mice with high expression levels of mutant SOD1G93A [29], whereas the genetic fusion of TTC with GDNF increased survival by 9 days and mitigated symptoms in ALS animal models [30].