dementia ͉ motor neuron disease ͉ neurodegeneration ͉ protein aggregation F TLD is a relatively common cause of dementia among patients with onset before 65, typically manifesting with behavioral changes or language impairment due to degeneration of subpopulations of cortical neurons in the frontal, temporal and insular regions (1). By contrast, ALS presents with muscle weakness and spasticity due to degeneration of motor neurons in both layer 5 of cortex and in the spinal cord, resulting in death from respiratory failure in 3-5 years (2, 3). Interestingly, approximately 20% of patients with ALS also develop FTLD, and approximately 15% of FLTD patients also develop ALS (4, 5).The discovery that TDP-43 is present in cytoplasmic aggregates in both ALS and FTLD-U provided evidence that the two disorders may share a common underlying mechanism (6). TDP-43 is an RNA/DNA binding protein, implicated in regulation of alternative splicing of messenger RNA, RNA stability, and transcriptional control (7). The concept that TDP-43 can play a direct role in neurodegeneration was strengthened by recent reports that dominantly inherited missense mutations in TDP-43 are found in patients with familial ALS (8-12). Mutations in TDP-43 associated with ALS cluster in the C-terminal glycine-rich region, which is involved in protein-protein interactions between TDP-43 and other heterogeneous nuclear ribonuclear proteins (hnRNPs) (13). Furthermore, C-terminal fragments of TDP-43 are observed selectively in ALS and FTLD-U tissues, suggesting that proteolytic cleavage of TDP-43 leads to protein aggregation or another toxic property (6). Therefore, several putative mechanisms of TDP-43 induced neurodegeneration are currently under investigation, including toxic protein aggregation, and/or disruption of normal TDP-43 RNA/DNA binding protein function. Here we report a mouse model of TDP-43 induced neurodegeneration which recapitulates key features of ALS and FTLD-U, including ubiquitin aggregate pathology with selective vulnerability of cortical projection neurons and spinal motor neurons, but without the presence of TDP-43 aggregates. Together with recent reports of mutations in another RNA binding protein (FUS/TLS) in familial ALS (14, 15), this supports that altered RNA-binding protein function (rather than toxic aggregation of TDP-43) likely plays a central and unexpected role in ALS pathogenesis.