C9 and Motor neuron atrophy: Furthermore, in mouse models, C9‐KO or C9‐HRE and the presence of RNA foci and DPRs have been shown to be insufficient to cause motor neuron degeneration (Koppers et al. 2015; O'Rourke et al. 2015), although loss of C9ORF72 can lead to synaptic dysfunction and excitotoxicity (Xiao et al. 2019).