Factor of infectivity and pathogenicity, required for optimal virus replication. Alters numerous pathways of T-lymphocyte function and down-regulates immunity surface molecules in order to evade host defense and increase viral infectivity. Alters the functionality of other immunity cells, like dendritic cells, monocytes/macrophages and NK cells. One of the earliest and most abundantly expressed viral proteins (By similarity). In infected CD4(+) T-lymphocytes, down-regulates the surface MHC-I, mature MHC-II, CD4, CD28 and probably other immunity surface molecules. In consequence infected cells are masked for immune recognition by cytotoxic T-lymphocytes. Decreasing the number of immune receptors also prevents reinfection by more HIV particles (superinfection). Bypasses host T-cell signaling by inducing a transcriptional program nearly identical to that of anti-CD3 cell activation. Interaction with TCR-zeta chain up-regulates the Fas ligand (FasL). Increasing surface FasL molecules and decreasing surface MHC-I molecules on infected CD4(+) cells send attacking cytotoxic CD8+ T-lymphocytes into apoptosis (By similarity). Plays a role in optimizing the host cell environment for viral replication without causing cell death by apoptosis. Protects the infected cells from apoptosis in order to keep them alive until the next virus generation is ready to strike (By similarity). Homodimer. Predominantly found in the paranuclear area, probably in the TGN. Correct localization requires PACS1. Also associates with the inner plasma membrane through its N-terminal domain. Nef stimulates its own export via the release of exosomes. Also incorporated in virions at a rate of about 10 molecules per virion, where it is cleaved (By similarity). The N-terminal domain is composed of the N-myristoyl glycine and of a cluster of positively charged amino acids. It is required for inner plasma membrane targeting of Nef and virion incorporation, and thereby for infectivity. This domain is also involved in binding to p53 (By similarity). The SH3-binding domain constituted of PxxP motifs mediates binding to several Src family proteins thereby regulating their tyrosine kinase activity. Belongs to the lentivirus primate group Nef protein family.