C9orf72 and amyotrophic lateral sclerosis: Here, we show that expression of the C9orf72 mutation in astrocytes recapitulates key aspects of C9orf72‐related ALS pathology and directly results in physiological dysfunction of control and C9orf72 MNs upon co‐culture, thus highlighting both cell‐autonomous astrocyte pathology and non‐cell autonomous MN pathophysiology.