We have previously demonstrated that the human T-cell leukemia virus type 1 (HTLV-1) Tax oncoprotein represses the trans-activation function of p53 tumor suppressor protein. Recently, several proteins with sequence homology to p53 have been identified. In this study, we demonstrated that Tax represses the transactivation functions of p73α, p73β, and p51A, the p53-related proteins, as well as p53. Moreover, a mutant Tax of coactivator CBP-binding site (K88A), which activated NF-κB but not CREB pathway, could not repress the p73 nor p51 trans-activation functions, indicating that CBP-binding domain of Tax is essential for the suppression of their functions. Using proteins of Gal4-fused N-terminal region of p73 and p51, we showed that Tax-mediated inactivation of p73 or p51 requires for their N-terminal trans-activation domains. Furthermore, only the putative N-terminal trans-activation domains of them did not have enough transcriptional activities and their adjacent regions are essential for their full transactivation, suggesting the existence of their second transactivation subdomains. Thus, HTLV-1 Tax inactivated the p53-related proteins through their N-terminal transactivation domains.