C9orf72 and motor neuron disorder: We did not find any significant differences in the CSF levels of APP‐derived peptides between participants with increased certainty of underlying FTLD‐Tau (n = 30, progressive supranuclear palsy‐corticobasal degeneration spectrum [PSP‐CBD], and non‐fluent variant of primary progressive aphasia [nfaPPA] participants) and FTLD‐TDP (n = 21, including patients with semantic variant PPA, patients with motor neuron disease and carriers of C9orf72, GRN, and VCP mutations).