A common genetic cause of Amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) is expansion of the intronic hexanucleotide repeat (GGGGCC)n in C9orf72. Here the authors reveal that the RNA (GGGGCC)n expansion repeat associated with ALS/FTD can generate condensates in the absence of proteins, highlighting the potential relevance of targeting RNA-structures to treat neurodegenerative diseases. Here, C9orf72 is linked to frontotemporal dementia.