Interestingly, lower levels of CSF MERTK but not AXL differentiated not only symptomatic from presymptomatic GRN +/− carriers, but also symptomatic from pre-symptomatic for MAPT and C9ORF72 mutation carriers as well, suggesting that it may be a biomarker of conversion to symptomatic FTD. This evidence concerns the gene AXL and frontotemporal dementia.