C9orf72 and fragile X-associated tremor/ataxia syndrome: Here, we utilized HCR to detect RNA foci associated with G4C2 repeat expansions in C9orf72 that are the most common genetic cause of ALS and FTD and CGG repeats associated with Fragile X disorders such as Fragile X-associated tremor/ataxia syndrome (FXTAS) [14, 28, 56].