CD4 and viral infectious disease: Our results demonstrated that: 1) envelope PtdSer can mediate HIV-1 binding via the host’s PtdSer-binding molecule; 2) binding mediated by envelope PtdSer cannot mediate virus infection without expression of CD4 and co-receptors on target cells, most likely due to the lack of activation of fusion activity of gp160; and 3) envelope PtdSer can facilitate HIV-1 infection of CD4 and co-receptor positive cells by facilitating the virus binding step (raw data not shown but available with detailed materials and methods from the corresponding author per request) (summarized in Fig. 2).