The "Two-Hit" Model of Retrovirus-Induced Leukemogenesis. (A) HTLV-1 infection of CD34+ hematopoietic progenitor and stem cells (HP/HSCs) leads to the development of Adult T-cell leukemia/lymphoma (ATLL). (B) FLV infection of erythroid progenitors leads to erythroleukemia. (C) M-MuLV infection of pro-T cells leads to T-cell lymphoma. The dotted line indicates the separation between the early and late phase of infection.