Cover photograph (Copyright © 2001, American Society for Microbiology. All rights reserved.): Binding of Fabs of 3-specific monoclonal antibody 4F2 to virions of reovirus type 3 strain Dearing. The atomic coordinates of both a prototype Fab (Jeffrey et al., Nat. Struct. Biol. 2:466-471, 1995) (top) and reovirus outer-capsid protein 3 (Olland et al., EMBO J. 20:979-989, 2001) (bottom) were placed into the density of virions bound by 4F2 Fabs (mesh) determined from electron cryomicroscopy and three-dimensional image analysis. The atomic coordinates of Fab and 3 were placed by using the Situs program package and displayed using the Ribbons program in gold and red, respectively. The complementarity-determining region loops of the fitted Fab were in close proximity to a virion-distal -helix, which contains amino acid 116 (indicated by an arrow). This amino acid lies directly beneath the bound Fab and is in close (~5 Å) contact with the fitted Fab coordinates. Comparative sequence analysis of reovirus field isolate strains indicates that sequence variability at residue 116 influences the capacity of monoclonal antibody 4F2 to bind 3 (Kedl et al., J. Virol. 69:552-559, 1995). (See related article in July 2001: vol. 75, issue 14, p. 6625.)