MAPT and frontotemporal dementia: The function and neuronal expression pattern of these different tau isoforms remain to be elucidated, although correct splicing seems to be necessary to keep neurons functional, as unbalanced 4R:3R tau ratios are linked with neurodegenerative disorders such as FTD and Huntington disease (Hutton et al., 1998, Fernandez-Nogales et al., 2014, Fernandez-Nogales et al., 2016).