Adult T-cell leukemia/lymphoma (ATL) is an aggressive and incurable malignancy of mature CD3+, CD4+, CD25+, CD7– T cells caused by human T-cell lymphotropic virus type-1 (HTLV-1), which was initially described in 1977 by Takatsuki (2005). The gene discussed is CD4; the disease is adult T-cell leukemia/lymphoma.