Abstract: Knock down or other inhibition of C9ORF72 expression provides a method of treating cancer, in particular cancers susceptible to PARP inhibition such as glioblastoma. Thus, agents that target C9orf72, such as inhibitory oligonucleotides or antibodies can be used to treat cancers. A specific embodiment includes methods for treating cancers by administrating such agents, such as an inhibitory oligonucleotide that targets C9orf72. Also disclosed are methods that involve the co-administration of a PARP inhibitor and an agent that targets C9ORF72.