C9orf72 and amyotrophic lateral sclerosis: Dipeptide levels are stable in longitudinal testing, but in vitro experiments with antisense constructs show that extracellular levels are a good reflection of the degree of silencing of the C9orf72 repeat RNA and therefore mark an important step towards the development of pre-clinical and treatment response assays for this important sub-group of ALS-FTD [58].