The thymus is an early site for HIV-1 infection: CD4 is expressed not only on mature (CD3+CD4+CD8−) T cells, but also on less mature (CD3−/+ CD4+CD8+) thymocytes and intrathymic T progenitor (ITTP, CD3−CD4+CD8−) cells (Meissner et al., 2003). Here, CD4 is linked to HIV-1 infection.