C9orf72 and frontotemporal dementia: The idea of c9ALS/FTD as a disorder mediated by an accumulation of several minor disruptions, rather than a few isolated RNA misprocessing events as has been described for myotonic dystrophy type 1 (Timchenko et al., 2001; Jiang et al., 2004), may in part account for the extreme variability in clinical phenotypes exhibited by C9orf72 expansion carriers.