Human T-cell leukemia virus type 1 (HTLV-1) can cause adult T-cell leukemia/lymphoma (ATLL) and inflammatory diseases such as HTLV-1–associated myelopathy/tropical spastic paraparesis (HAM/TSP). My research group established a transgenic (Tg) mouse model of HTLV-1 infection using the distal promoter of Lck to induce tax expression in mature thymocytes and peripheral T lymphocytes. The major disease phenotypes in this model were mature T cell leukemia/lymphoma, similar to ATLL, and inflammatory arthropathy. While expanding Tax-Tg mouse colony, my group found that about 2% of the Tg mice developed Stevens–Johnson syndrome (SJS) and toxic epidermal necrolysis (TEN)-like disease. SJS/TEN-like lesions were characterized by a rash and diffuse exfoliation of large areas of the skin, similar to second-degree burns. The pathology of Tg mice with SJS/TEN-like disease included epidermal necrosis and detachment at the dermoepidermal junction. Serum soluble Fas ligand levels were significantly increased in Tax-Tg mice with SJS/TEN-like disease.