Septin9 (Sept9) is a member of the filament-forming septin family of structural 
proteins and is associated with a variety of cancers and with hereditary 
neuralgic amyotrophy. We have generated mice with constitutive and conditional 
Sept9 knockout alleles. Homozygous deletion of Sept9 results in embryonic 
lethality around day 10 of gestation whereas mice homozygous for the conditional 
allele develop normally. Here we report the consequences of homozygous loss of 
Sept9 in immortalized murine embryonic fibroblasts. Proliferation rate was not 
changed but cells without Sept9 had an altered morphology compared to normal 
cells, particularly under low serum stress. Abnormal, fragmented, and multiple 
nuclei were more frequent in cells without Sept9. Cell migration, as measured by 
gap-filling and filter-invasion assays, was impaired, but individual cells did 
not move less than wild-type cells. Sept9 knockout cells showed a reduced 
resistance to hypo-osmotic stress. Stress fiber and vinculin staining at focal 
adhesion points was less prominent. Long septin filaments stained for Sept7 
disappeared. Instead, staining was found in short, often curved filaments and 
rings. Furthermore, Sept7 was no longer localized to the mitotic spindle. 
Together, these data reveal the importance of Sept9 for septin filament 
formation and general cell stability.