Nef is a 27-34 kD myristoylated protein unique to primate lentiviruses. A functional Nef gene is important for development of high viremia and simian AIDS in SIV infected rhesus macaques. Notably, animals infected with Nef-deleted attenuated viruses are resistant to subsequent challenge with pathogenic wild-type viruses. A critical role for Nef in development of AIDS in humans has been suggested by the observation that some individuals with a long-term nonprogressive HIV-1 infection (persons who show no clinical or immunological signs of immunodeficiency despite being HIV seropositive for over a decade) are infected with viruses carrying naturally occurring Nef deletions. The mechanism of Nef action remains incompletely understood, but multiple lines of evidence point out to a role in modulation of cellular signaling pathways via physical and functional interactions with host cell protein kinases. These findings will be discussed in the following, preceded by a short introduction into the role of Nef in cell biology of HIV infection, which is intended to serve as a critical review of our current understanding on this enigmatic issue rather than a comprehensive review of the literature.