Trans-activation response DNA-binding protein (TDP-43) accumulation is the major component of ubiquitinated protein inclusions found in patients with amyotrophic lateral sclerosis, and frontotemporal lobar degeneration with TDP-43 positive ubiquitinated inclusions, recently relabelled the 'TDP-43 proteinopathies'. TDP-43 is predominantly located in the nucleus, however, in disease it mislocalizes to the cytoplasm where it aggregates to form hallmark pathological inclusions. The identification of TDP-43 mutations in familial and sporadic amyotrophic lateral sclerosis cases confirms its pathogenic role; but it is wild-type TDP-43 that is deposited in the vast majority of TDP-43 proteinopathies, implicating other unknown factors for its mislocalization and aggregation. One such mechanism may be defective nuclear import of TDP-43 protein, as a disruption of its nuclear localization signal leads to mislocalization and aggregation of TDP-43 in the cytoplasm. In order to explore the factors that regulate the nuclear import of TDP-43, we used a small interfering RNA library to silence 82 proteins involved in nuclear transport and found that knockdowns of karyopherin-beta 1 and cellular apoptosis susceptibility protein resulted in marked cytoplasmic accumulation of TDP-43. In glutathione S-transferase pull-down assays, TDP-43 bound to karyopherin-alpha s, thereby confirming the classical nuclear import pathway for the import of TDP-43. Analysis of the expression of chosen nuclear import factors in post-mortem brain samples from patients with TDP-43 positive frontotemporal lobar degeneration, and spinal cord samples from patients with amyotrophic lateral sclerosis, revealed a considerable reduction in expression of cellular apoptosis susceptibility protein in frontotemporal lobar degeneration. We propose that cellular apoptosis susceptibility protein associated defective nuclear transport may play a mechanistic role in the pathogenesis of the TDP-43 positive frontotemporal lobar degeneration.