Clustering for the kill Cluster of differentiation 20 (CD20) is a membrane protein that defines most B cell populations and is the target of therapeutic antibodies to treat malignancies and autoimmune disorders. Rougé et al. present the structure of CD20 bound to the antibody rituximab that activates the complement system to kill B cells. CD20 forms a dimer and each monomer binds one rituximab antigen-binding fragment (Fab) to give 2:2 stoichiometry. The compact packing between Fab arms and CD20 gives rise to circular assemblies with a diameter similar to that of antibody hexamers known to recruit the first component of the complement cascade. Science , this issue p. 1224