Mucosal transmission of HIV is a major force driving expansion of the global AIDS epidemic. Understanding the initial events in the establishment of infection in a new host is key to developing an effective vaccine and microbicides.Our main focus is to understand HIV transmission: what cells HIV initially infects in the genital mucosa and what HIV features are critical for viral transmission from one person to another. We are particularly interested in the envelope protein encoded by HIV and how it mediates fusion, the initial step of the viral life cycle. We developed a fluorometric assay to identify the most susceptible cells in vivo and characterize the strains of HIV circulating in the population