Schematic presentation of the clonal progression and hierarchical structure of HTLV-1–infected T cells. A clone that is defined by the integration site contains subclones with different genetic abnormalities. Among these subclones, there is a hierarchical structure where T_SCM cells behave as ATL-initiating stem cells. T_CM, CD45RO⁺ central memory T cells; T_EM, CD45RO⁺ effector memory T cells; T_N, CD45RA⁺ naive T cells.