Objective To explore the regulatory effects of HTLV-1 ( human T-cell leukemia virus type 1 ) Tax protein on the expression of HMGB 1 ( high mobility group box 1 ) gene in T cells .Methods Total RNA and protein were extracted from Tax +-T cells ( TaxP ) , Tax--T cells ( TaxN ) and Jurkat cells which were stably transfected with pCMV-Tax and pCMV-Neo, respectively.Then, the expression levels of HMGB1 mRNA and protein in different CD 4+T cells were analyzed by real-time PCR and Western blot (WB).By using liposome-mediated method, pGL3-HMGB1-luc reporter genes and pGL3-neo-luc were tran-siently transfected into TaxP and TaxN cells and the basal transcriptional activity was observed in different T cells.Additionally, pCMV-Tax and pGL3-HMGB1-luc reporter genes were also co-transfected into Jurkat cells and the regulatory effects of Tax protein on HMGB 1 gene was detected .The chromatin immunoprecipi-tation (ChIP) assay was used to identify HMGB1 genomic sites directly targeted by Tax .Results The ex-pression levels of HMGB1 mRNA and protein in Tax+-T cells ( TaxP) were higher than those in Tax--T cells (TaxN).The transcription regulation trends for HMGB1 gene in TaxN and TaxP cells were similar but not identical in diverse T cells.pHLuc3 (containing -504-+83 HMGB1) showed the highest transcriptional ac-tivity of HMGB1 gene in both TaxP and TaxN cells , but HMGB1 transcriptional activity of pHLuc 6 in TaxP cells was significantly stronger than that in TaxN cells .Luciferase assays also showed that Tax protein promo-ted the transcription of HMGB1 gene in a dose-dependent manner .The ChIP assay further confirmed that Tax protein enriched at the HMGB1 region of -1163--1043.Conclusion The region of nt -504--383 is essen-tial for the basal promoter activity of -1163-+83 HMGB1 gene originated from pHLuc 6 reporter plasmid , and Tax protein enriched probably at the HMGB 1 site of -1163--1043 enhances HMGB1 transcription.