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In mammalian cells, actin dynamics is tightly controlled through small GTPases of the Rho family, WASP/Scar proteins and the Arp2/3 complex. We employed Cre/loxP-mediated gene targeting to disrupt the ubiquitously expressed N-WASP in the mouse germline, which led to embryonic lethality. To elucidate the role of N-WASP at the cellular level, we immortalized embryonic fibroblasts and selected various N-WASP-defective cell lines. These fibroblasts showed no apparent morphological alterations and were highly responsive to the induction of filopodia, but failed to support the motility of Shigella flexneri. In addition, enteropathogenic Escherichia coli were incapable of inducing the formation of actin pedestals in N-WASP-defective cells. Our results prove the essential role of this protein for actin cytoskeletal changes induced by these bacterial pathogens in vivo and in addition show for the first time that N-WASP is dispensible for filopodia formation.