GRN and amyotrophic lateral sclerosis: The majority of these studies have examined an undifferentiated FTD cohort, not stratified by genetic or pathological subtype, but in the small studies that have investigated specific forms of FTD, there is some evidence for a particular role of microglial activation in those with progranulin (GRN) mutations12, 16 and those with associated amyotrophic lateral sclerosis (ALS).14