Lehmer et al (2017) show that CSF levels of neurofilaments (Nfl and pNfH), markers of axonal damage, are raised in symptomatic but not asymptomatic C9orf72 mutation carriers, in agreement with recent studies of neurofilaments as ALS diagnostic and prognostic biomarkers (Fig 1). The gene discussed is C9orf72; the disease is amyotrophic lateral sclerosis.