The G 4 C 2 repeat expansion in the C9orf72 gene is the most common genetic cause of Amyotrophic Lateral Sclerosis and Frontotemporal Dementia. Many studies suggest that dipeptide repeat proteins produced from this repeat are toxic, yet, the contribution of repeat RNA toxicity is under investigated and even less is known regarding the pathogenicity of antisense repeat RNA. Recently, two clinical trials targeting G 4 C 2 (sense) repeat RNA via antisense oligonucleotide failed despite a robust decrease in sense encoded dipeptide repeat proteins demonstrating target engagement. Here, in this brief report, we show that G 2 C 4 antisense, but not G 4 C 2 sense, repeat RNA is sufficient to induce TDP-43 dysfunction in induced pluripotent stem cell (iPSC) derived neurons (iPSNs). Unexpectedly, only G 2 C 4 , but not G 4 C 2 sense strand targeting, ASOs mitigate deficits in TDP-43 function in authentic C9orf72 ALS/FTD patient iPSNs. Collectively, our data suggest that the G 2 C 4 antisense repeat RNA may be an important therapeutic target and provide insights into a possible explanation for the recent G 4 C 2 ASO clinical trial failure.