Schematic description of clonal progression and phenotypic changes. HTLV-1–infected T cells and ATL cells in vivo are now available for molecular analyses. Accumulating data indicate that epigenetic abnormalities occur early during leukemogenesis, as untransformed HTLV-1–infected cells show evidence of epigenetic abnormalities that are observed in ATL cells as well. The extent of clonality during HTLV-infection and progression to ATL have been characterized in detail by recent studies that provide information on clonal progression based on integration sites.