The disclosure relates to oligomers capable of targeting RNA expressed from the human C9ORF72 gene containing a pathogenic hexanucleotide repeat expansion. Such oligomers are useful for, among other things, reducing or eliminating C9ORF72 RNA and/or proteins translated therefrom, and treating or preventing diseases or disorders caused by, or associated with, hexanucleotide repeat expansion, including familial frontotemporal dementia (FTD) and familial amyotrophic lateral sclerosis (ALS).