Certain conditions can enhance HIV infection of naive CD4+ T cells, including transmission in a tonsil lymphoid tissue microenvironment ex vivo (68, 69) or in coculture with monocyte-derived DCs (MDDCs) in vitro (70), although the latter condition was only shown to enhance infection by CXCR4-tropic (X4), but not CCR5-tropic (R5), virus. This evidence concerns the gene CXCR4 and infection.