Although the HTLV family of retroviruses is distinct from the HIV family, both groups of viruses infect human T4 (CD4) lymphocytes preferentially and alter the host's T-cell functions. Whereas HTLV-I or HTLV-II primarily induce transformation and proliferation of T Lymphocytes and cause T-cell lymphoma/leukemia in humans, HIV and HIV-2 induce T-cell cytopathology that leads to depletion of CD4 cells.