To exclude the possibility of reduced mitochondrial DNA driving the transcriptional signature in C9orf72-ALS MNs, we next quantified mitochondrial DNA copy number through a BaseScopeTM DNA in situ hybridisation technique that uses a probe targeting the antisense strand of the MT-CO1 gene (see Materials and Methods). The gene discussed is C9orf72; the disease is amyotrophic lateral sclerosis.