C9orf72 and amyotrophic lateral sclerosis: In addition to the important role of p53 in cancer, p53 stabilization and activation by C9orf72 poly-PR seem to be key mechanisms causing the neurodegenerative phenotype in primary cortical neurons expressing C9orf72 poly-PR, iPSCs from ALS patients with C9orf72 GGGGCC, transgenic fly expressing C9orf72 GGGGCC, and transgenic mouse expressing C9orf72 poly-PR [40].