Here, the authors demonstrate that muscleblind (Mbl) is a novel modifier of FUS-associated ALS, with knockdown of endogenous Mbl suppressing neuromuscular junction defects and motor dysfunctions associated with FUS expression in Drosophila, as well as restoring reduced SMN protein levels in mammalian neuronal and human iPSC-derived motor neurons. This evidence concerns the gene SMN1 and amyotrophic lateral sclerosis.