To prepare future treatment trials in RFC1 disease, we here leveraged a large cross-European multicenter ataxia cohort to (1) map its full phenotypic spectrum and evolution beyond CANVAS (using both a cohort expected to be enriched for RFC1 disease and an independent cohort of unselected patients with late-onset ataxia); (2) single out discriminative features of patients with RFC1-positive and RFC1-negative ataxia; and (3) map the natural disease history, including first piloting quantitative longitudinal disease progression data and preliminary sample size calculations. The gene discussed is RFC1; the disease is Ataxia.