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Abstract: The BTas protein of bovine foamy virus (BFV) is a 249-amino-acid nuclear regulatory protein which transactivates viral gene expression directed by the long terminal repeat promoter (LTR) and the internal promoter (IP). Here, we demonstrate the BTas protein forms a dimeric complex in mammalian cells by using mammalian two hybrid systems and cross-linking assay. Functional analyses with deletion mutants reveal that the region of 46–62aa is essential for dimer formation. Furthermore, our results show that deleting the dimerization region of BTas did not affect the localization of BTas, but that it did result in the loss of its transactivational activity on the LTR and IP. Furthermore, BTas (Δ46–62aa) retained binding ability to the LTR and IP similar to that of the wild-type BTas. These data suggest the dimerization region is necessary for the transactivational function of BTas and is crucial to the replication of BFV. [Copyright &y& Elsevier]