Investigators found elevated levels of tau and a phosphorylated form of the tau protein in brain tissue from deceased people who had amyotrophic lateral sclerosis (ALS) and carried a mutation in the C9orf72 gene. The team also identified new genetic mutations in the tau gene that may exacerbate ALS onset or progression. Tau in the cerebrospinal fluid may be a viable biomarker for ALS because levels correlate with disease progression.