A number of studies show that mitochondrial DNA (mtDNA) depletion and attendant activation of retrograde signaling induces tumor progression. We have reported previously that activation of a novel nuclear factor-Kappa B pathway is critical for the propagation of mitochondrial retrograde signaling, which induces both phenotypic and morphological changes in C2C12 myoblasts and A549 lung carcinoma cells. In this study, we investigated the role of stress-induced nuclear factor-Kappa B in tumor progression in xenotransplanted mice. We used a retroviral system for the inducible expression of small interfering RNA against IkBα and IkBβ mRNAs. Expression of small interfering RNA against IkBβ markedly impaired tumor growth and invasive ability of mtDNA-depleted C2C12 myoblasts and also thwarted anchorage-independent growth of the cells. Knockdown of IkBα mRNA, however, did not have any modulatory effect in this cell system. Moreover, expression of small interfering RNA against IkBβ reduced the expression of marker genes for retrograde signaling and tumor growth in xenografts of mtDNA-depleted cells. Our findings demonstrate that IkBβ is a master regulator of mitochondrial retrograde signaling pathway and that the retrograde signaling plays a role in tumor growth in vivo. In this regard, IkBβ supports the tumorigenic potential of mtDNA-depleted C2C12 cells.