GRN and amyotrophic lateral sclerosis: Moreover, while FTD-ALS is pathologically homogenous, invariably being associated with TDP-43 pathology,5 half of FTD cases without ALS have alternative pathologies: tau or fused-in-sarcoma.12 Furthermore, of the three main genes implicated in FTD: C9ORF72, PGRN and MAPT, only C9ORF72 is associated with FTD-ALS.