Repeat expansions in C9orf72 are a frequent cause of amyotrophic lateral sclerosis and frontotemporal dementia. A new study reports that individuals with these repeat expansions show abnormalities in gyrification (cortical folding) before symptoms appear. Caverzasi et al. investigated patterns of cortical gyrification and thickness in 15 presymptomatic C9orf72 expansion carriers and 67 age-matched and sex-matched controls. Presymptomatic C9orf72 expansion carriers showed a lower local gyrification index in left frontal and right parieto-occipital regions compared with controls. Gyrification measurements did not correlate with cortical thickness measurements in presymptomatic carriers or healthy controls, suggesting that gyrification is a novel grey matter metric that is distinct from cortical thickness.