Mutations in fused in sarcoma (FUS) are a cause of familial amyotrophic lateral 
sclerosis (fALS). Patients carrying point mutations in the C-terminus of FUS 
show neuronal cytoplasmic FUS-positive inclusions, whereas in healthy controls, 
FUS is predominantly nuclear. Cytoplasmic FUS inclusions have also been 
identified in a subset of frontotemporal lobar degeneration (FTLD-FUS). We show 
that a non-classical PY nuclear localization signal (NLS) in the C-terminus of 
FUS is necessary for nuclear import. The majority of fALS-associated mutations 
occur within the NLS and impair nuclear import to a degree that correlates with 
the age of disease onset. This presents the first case of disease-causing 
mutations within a PY-NLS. Nuclear import of FUS is dependent on Transportin, 
and interference with this transport pathway leads to cytoplasmic redistribution 
and recruitment of FUS into stress granules. Moreover, proteins known to be 
stress granule markers co-deposit with inclusions in fALS and FTLD-FUS patients, 
implicating stress granule formation in the pathogenesis of these diseases. We 
propose that two pathological hits, namely nuclear import defects and cellular 
stress, are involved in the pathogenesis of FUS-opathies.