Gp120 Binds Cooperatively to Several Biologically Relevant Glycosphingolipids: Quantitative Measurements at Equilibrium by Total Internal Reflection Fluorescence Microscopy This research was supported by the NIH (AI40359-02), the NSF (CHE-9726132 and CHE-9623583), Eli Lilly (JGH), and the Alfred P. Sloan Foundation (J.G.H.). K.D.M. gratefully acknowledges receipt of the University of Arizona Dean's Fellowship and the Department of Chemistry Carl S. Marvel Fellowship. We thank Ying-Mei Gu for performing the streptavidin adsorption measurements.