Infection of the neural progenitor cells by neurovirulent viruses such as Borna disease virus (BDV) has been shown in vitro to impair cellular development of progenitor cells [20]. A previous study also demonstrated that BDV infection decreases cell numbers during neural differentiation [20]. In the present study, cell death was observed predominantly in H5N1-infected groups, whereas the phenotype of the remaining hNPCs following H5N1 virus infection remained unaltered. These results could be explained by the facts that the differentiation protocol was not introduced in the present study. Moreover, the differentiation stage of these cells before cell death is unknown. Although we believe that the decrease in cell numbers were not due to experimental artifact, since the same number of cells were seeded into both infected- and mock-infected groups prior to virus infection. Though it is not known whether such an infection could induce differentiation of the hNPCs and, at the same time, triggers cell death of these differentiating cells. Still, it remains to be determined whether susceptible infection of H5N1 virus depends on the differentiation stages of hNPCs, or the differentiation capacity of hNPCs is triggered by H5N1 virus. Previous studies have shown that viruses can induce cellular dedifferentiation by either direct lytic effect or cellular dysregulation following exposure to pro- and inflammatory cytokines [20,21,39,49]. Taken together, the present study showed that hNPCs support H5N1 virus infection in vitro. Infection resulted in significant cell losses and up-regulation of TNF-α pro-inflammatory cytokine. Moreover, phenotypic characterization of hNPCs was unaltered following H5N1 virus infection. Although our data did not demonstrate the dysregulation of hNPC development during acute H5N1 virus infection, it remains to be determined whether H5N1 virus or other low pathogenic strains of influenza viruses can deregulate neurogenesis or astrogliogenesis during the acute and chronic phase of infection. The results of the present study further support the role of at least one type of influenza virus in inducing encephalopathy.