The HTLV-1 virus is present worldwide.  In less than 5% of infected individuals, this virus causes the development of Adult T-cell Leukemia/Lymphoma (ATLL).  Unlike HTLV-1, the related HTLV-2 virus does not cause any type of cancer and no satisfactory explanations have been provided for this difference.  Recent studies have led to the discovery and description of an unexpected gene in the genome of HTLV-1 (termed HBZ).  The expression of this gene is atypical for this class of virus. In our laboratory, we have obtained results showing that a similar gene (termed ASP-2) is also expressed in HTLV-2.  As recent results suggest that HBZ could play a role in ATLL development, functional differences between both HBZ and ASP-2 proteins might contribute to these diverse pathological outcomes.  Our study will mainly compare how these proteins differ in terms of function, cellular localization and their impact on cell proliferation. The body of results obtained from this study will allow a better understanding of HTLV-1 and HTLV-2 replication and assess the role played by HBZ in the development of ATLL.