The inhibitor of NF-kappaB (IkappaB) family of proteins is believed to regulate 
NF-kappaB activity by cytoplasmic sequestration. We show that in cells depleted 
of IkappaBalpha, IkappaBbeta and IkappaBepsilon proteins, a small fraction of 
p65 binds DNA and leads to constitutive activation of NF-kappaB target genes, 
even without stimulation, whereas most of the p65 remains cytoplasmic. These 
results indicate that although IkappaBalpha, IkappaBbeta and IkappaBepsilon 
proteins could be dispensable for cytoplasmic retention of NF-kappaB, they are 
essential for preventing NF-kappaB-dependent gene expression in the basal state. 
We also show that in the absence of IkappaBalpha, IkappaBbeta and IkappaBepsilon 
proteins, cytoplasmic retention of NF-kappaB by other cellular proteins renders 
the pathway unresponsive to activation.