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Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disorder characterised by the progressive degeneration of brain and spinal cord motor neurons. Ubiquitin–proteasome system (UPS) dysfunction and oxidative stress have been implicated in ALS pathogenesis. However, it is unknown whether the defects in these pathways extend to non-neuronal tissues such as fibroblasts. Fibroblasts, unlike neuronal tissue, are readily available and may hold potential for short-term, rapid diagnostic and prognostic purposes. We investigated whether primary skin fibroblasts from ALS patients share, or can be manipulated to develop, functional and pathological abnormalities seen in affected neuronal cells. We inhibited UPS function and induced oxidative stress in the fibroblasts and found that ALS-related cellular changes, such as aggregate formation and ubiquitination of ALS-associated proteins (TDP-43 and ubiquilin 2), can be reproduced in these cells. Higher levels of TDP-43 ubiquitination, as evident by colocalization between TDP-43 and ubiquitin, were found in all six ALS cases compared to controls following extracellular insults. In contrast, colocalization between ubiquilin 2 and ubiquitin was not markedly different between ALS cases and control. A UPS reporter assay revealed UPS abnormalities in patient fibroblasts. Despite the presence of ALS-related cellular changes in the patient fibroblasts, no elevated toxicity was observed. This suggests that aggregate formation and colocalization of ALS-associated proteins may be insufficient alone to confer toxicity in fibroblasts used in the present study. Chronic exposure to ALS-linked stresses and the ALS-linked cellular pathologies may be necessary to breach an unknown threshold that triggers cell death.