The Nef protein of human immunodeficiency virus (HIV) is a crucial factor in viral pathogenesis. One of the roles of this multifunctional protein is to decrease the cell-surface expression of CD4, a component of the virus receptor complex, and of MHC class I molecules. New results indicate that Nef can link CD4 to endocytic clathrin adaptor complexes and might also modulate other steps in the endocytic pathway.