This analysis revealed three statistically significant associations: (1) Patients carrying the C9orf72 expansion significantly differed from non-carrier patients and showed serum BDNF levels comparable to control subjects; (2) BDNF levels in CSF were significantly higher in ALS patients with faster disease progression; (3) lower serum levels of Pro-BDNF were associated with a shorter survival. Here, BDNF is linked to amyotrophic lateral sclerosis.