Graphic to accompany the on-line abstract for "Use of the quartz crystal microbalance to monitor ligand-induced conformational rearrangements in HIV-1 envelope protein gp120," by Hyun-Su Lee, Mark Contarino, M. Umashankara, Arne Schön, Ernesto Freire, Amos B. Smith, III, Irwin M. Chaiken, and Lynn S. Penn