For NF-L, soma accumulations were already observed in SOD1 and C9orf72 yMNs (Supplementary Fig. 7a) and with time in culture these accumulations increased significantly in all 3 mutant ALS forms compared to controls (Fig. 3a, b) as shown by quantitative analyses (Fig. 3c, d; Supplementary Fig. 7c for individual data). Here, SOD1 is linked to amyotrophic lateral sclerosis.