In the case of tau protein, the involvement of several protein isoforms in patients’ fibrils (either by a combination of the 3R/4R isoforms, as in AD, or exclusively 3R, as in Pick’s disease, or 4R isoforms, as in corticobasal degeneration) is in part responsible of distinct folds of tau in tauopathies. The gene discussed is MAPT; the disease is frontotemporal dementia.