We show further that hiPSC-phMNs obtained from hiPSC lines derived from ALS patients with C9orf72 amplification, the most common familial ALS mutation, provide a disease-relevant experimental system to address existing ambiguities about the impact of C9orf72 amplification on MN survival28–31 and activity28,32,33. The gene discussed is C9orf72; the disease is amyotrophic lateral sclerosis.