HTLV-I is the causative agent of adult T-cell leukemia/lymphoma (ATL). HTLV-1 oncoprotein Tax is known to induce DNA damage, micronuclei formation, multinucleation, and DNA aneupoidy. The genomic instability induced by Tax is thought to play an important role in ATLL development. We have shown that the Tax can activate the anaphase promoting complex/cyclosome (APC/C), an E3 ubiquitin ligase that controls metaphase to anaphase transition and mitotic exit. APC/C activation by Tax leads to premature poly- ubiquitination and degradation of cell cycle regulators including cyclin A, cyclin B, securin, and Skp2. Skp2 is the substrate-targeting subunit of another E3 ligase known as SCFSkp2, which mediates the destruction of cyclin E/A-CDK2 inhibitor (CKI), p27. The degradation of Skp2 by APC/C as induced by Tax leads to SCFSkp2 inactivation and p27 stabilization. The mRNA level of another CKI, p21, also increases sharply as a result of promoter activation by Tax and mRNA stabilization due to Tax-induced APC/C activation. The massive surge in p21 and p27 in turn induces rapid senescence (Tax-IRS). Importantly, HeLa and SupT1 T cells infected by HTLV-1 arrest in senescence as well. By contrast, cells deficient in p21 and p27, such as HOS, escaped Tax- IRS and continue to divide after HTLV-1 infection or Tax expression. They however developed dramatic multinucleation and double-stranded DNA breaks, most likely as a result of constitutive APC/C activation. As might be expected, p27 is down-regulated and p21, mis-localized, in HTLV-1-transformed T (HTxT) cells which express Tax abundantly. The p21 and p27 deficiencies apparently allow HTxT cells to overcome Tax-induced senescence. Consistent with the idea that precocious activation of APC/C is deleterious to the cell, we recently found that the prototypic HTxT cell line, MT4, in addition to p21 and p27 deficiencies, expressed a much lower level of Cdh1, the substrate-targeting subunit of APC/C. Recent data from multiple labs indicate that premature APC/C activation through the depletion of Emi1, the S/G2 inhibitor of APC/C, disrupts Cdt1-geminin imbalance and promotes re-replication of DNA. The aberrant DNA replication (re-/hyper-replication) in turn causes multinucleation, double-stranded DNA breaks and DNA damage checkpoint activation. Emi1 depletion also results in premature degradation of Skp2 by APC/C, p27 stabilization, and senescence. Remarkably, the cell cycle phenotypes of Emi1 knockdown resemble those of Tax. These new findings validate our observations and raise many more interesting questions that will be addressed in the following three specific aims: aim 1, to elucidate the mechanism by which Tax activates APC/C; aim 2, to investigate the biological consequences of Tax-mediated APC/C activation; and aim 3, to determine how transformed T cells adapt to Tax expression.