To elucidate the disease mechanism(s) associated with C9ALS/FTD, transgenic mice have been generated in which one or both C9orf72 alleles were inactivated [11], or in which hundreds (≥ 450) of patient-derived G4C2 hexanucleotide repeat expansions were expressed through bacterial artificial chromosomes (BACs) [11–14]. Here, C9orf72 is linked to frontotemporal dementia.