Two teams have structurally analyzed an antibody with unusually potent and broad activity against HIV strains, reviving hopes for an effective vaccine, which is urgently needed to fight AIDS. Last year, researchers discovered two antibodies that neutralize 70 to 85% of global HIV strains and have much greater potency than previous anti-HIV antibodies. The crystal structure of one of the antibodies, PG16, has now been determined independently by groups led by Dennis R. Burton and Ian A. Wilson of Scripps Research Institute (Proc. Natl. Acad. Sci. USA, DOI: 10.1073/pnas.1004600107) and Peter D. Kwong of the National Institute of Allergy & Infectious Diseases (J. Virol., DOI: 10.1128/JVI.00966-10). PG16 turns out to have a novel antigen-binding site—a “hammerhead” that extends far beyond the antibody surface and has an unusual sulfotyrosine residue. The work also shows that 20% of PG16’s sequence is altered during the maturation process and that these changes contribute to the breadth and potency of the antibody’s response. The structure should help explain how PG16 penetrates HIV’s sugar-protected surface to disrupt viral infection of human immune cells and could aid the search for vaccines.