Peripheral T-cell lymphoma (PTCL) is a neoplastc disease of peripheral T-lymphocytes/NK cells, including PTCL unspecified, anaplastic large T-cell lymphoma (ALCL), IBL-like T-cell lymphoma (AILD), intestinal T-cell lymphoma (ITCL) and adult T-cell leukemia/lymphoma (ATL). The incidence of PTCL is relatively uncommon although its incidence shows significant variations in the geographical regions and racial populations. In Asia, its incidence is high due to HTLV-I infection in Japan. Molecular mechanisms of oncogenesis of PTCLs remain unknown. Therefore, analyses of ATL will give us a clue to clarify the molecular mechanism.