We found a gradient in NfL levels in the ALS-FTD spectrum (Fig. 2) with highest levels in patients with ALS without FTD (median = 3093, IQR = [2107–4261] pg/mL) followed by those with ALS-FTD (median = 1386, IQR = [836–2731] pg/mL) p = 0.005, and those with FTD without motor neuron symptoms (median = 1240, IQR = [859–2378] pg/mL). Here, NEFL is linked to amyotrophic lateral sclerosis.