Nef is a early protein that appears to play a role in optimizing the host cell environment for viral replication without causing cell death by apoptosis. Nef enhances virus infectivity and pathogenicity. It down modulates surface MHC I molecules and internalized molecules are sequested to the trans-Golgi network. The number of cell surface CD4 antigen are decreased by interacting with the Src family kinase LCK thereby inducing LCK CD4 dissociation and by increasing clathrin-dependent endocytosis of this antigen to target it to lysosomal degradation.