Human T-cell leukemia virus type 1 (HTLV-1) is an oncogenic retrovirus that is the etiological agent of adult T-cell leukemia (ATL). The HTLV-1 basic leucine zipper factor (HBZ), which is encoded by the minus strand of the provirus, is constitutively expressed in all ATL patient cells and likely contributes to the development and maintenance of ATL. Furthermore, the overexpression of the myeloid cell leukemia 1 (MCL1) protein is frequently observed in hematological cancers as well as several other types of cancers. Here, we found that the expression of HBZ in cells stabilized MCL1 protein expression and suppressed the MCL1-mediated release of cytochrome c from the mitochondria. This effect was mediated by inhibition of the ubiquitin-dependent degradation of MCL1. In a serial binding assay, HBZ interacted with cullin 1 (CUL1) through a head-to-tail interaction. The association between CUL1 and Skp1, which serves as the molecular scaffold for the components of SCF ubiquitin ligase complexes, was markedly repressed in the presence of HBZ. Mechanistic analysis indicated that HBZ abrogated the CUL1 association with Skp1, which in turn promoted the cellular expression of MCL1. This novel function of HBZ likely plays a role in the viral pathogenesis of HTLV-1 and provides important insights into our understanding of the development of ATL.H uman T-cell leukemia virus type 1 (HTLV-1) infects at least 5 million to 10 million people worldwide and is the causative agent of adult T-cell leukemia (ATL) (1-4). In most cases, HTLV-1 infection is transmitted through breast milk, blood cells, and dendritic cells in vivo (5). The majority of HTLV-1 carriers do not develop any significant clinical symptoms throughout their lives; however, approximately 5% of HTLV-1-infected subjects progress to ATL (6). Although ATL was discovered 40 years ago, there is still no effective treatment for this disease, in part because the underlying mechanisms of HTLV-1-mediated oncogenesis have not been fully elucidated.The HTLV-1 genome encodes three common retroviral structural and enzymatic proteins (gag, pol, and env proteins) and is flanked by long terminal repeats (LTR) at each end. There is a pX region located between the env gene and the 3= LTR (7). The plus strand of the pX region encodes regulatory and accessory proteins, including p12 I , p21 I , p13 II , p30 II , Rex, and Tax (7). It is well established that the Tax protein is a potent oncoprotein that either strongly activates or inactivates the transcription of target genes as well as interacts with numerous cellular factors that promote the survival and immortalization of HTLV-1-infected T cells (8-13). Interestingly, the Tax protein was detected in only 40% of ATL cases (14, 15) due to nonsense mutations, insertions, deletions, and epigenetic alterations, such as DNA methylation and histone modifications of the 5= LTR of the HTLV-1 provirus (16)(17)(18)(19). These studies suggested that Tax is required for the virus to enhance viral spreading during the earliest stage of HTLV-1 infec...