Fused-in-Sarcoma (FUS) gene encodes an RNA/DNA binding protein whose mutations are linked to amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD). This study shows that FUS functions in the neuronal DNA damage response by its recruitment to the site of DNA double-stranded breaks and by its interaction with histone deacetylase 1. The study also shows ALS/FTLD-associated mutant FUS is defective in DNA repair mechanism and that ALS/FTLD patients with FUS mutations have greater DNA damage.