CD4 and HIV-1 infection: Due to CD4 expression on nearly all thymocytes, the extent of thymic HIV-1 infection and subsequent thymic destruction is determined primarily by co-receptor tropism: Studies using SCID-hu Thy-Liv mice and human thymocyte tissue cultures show X4-tropic (CXCR4) HIV-1 strains result in rapid destruction of all thymocyte subpopulations, while R5-tropic (CCR5) HIV-1 strains result in slower destruction of stromal cells and mature thymocytes (Ye et al., 2004).