Early steps in virus replication require binding of the virus to its target cell, entry into the cell, and delivery of the viral genome into the cell in order to gain access to the cellular macromolecular synthesis machinery. This infectious process can only be initiated if the virus successfully attaches to a specific receptor on the plasma membrane of the target cell. Receptor specificity is one important factor for determining the target cell tropism of a virus. Picornaviruses are a group of small non-enveloped viruses belonging to a well- characterized virus family. The prototypic picornavirus consists of a singlestranded positive-sense RNA molecule that is encased by a protein capsid approximately 50Å thick and 300Å in diameter. The capsid is composed of 60 copies of four virus proteins, arranged in a T=1 icosahedral lattice. The three larger proteins (VP1-3) form the outer surface and share a common eight-antiparallel-beta-barrel structural motif. The smallest viral protein (VP4) is located at the interface between the capsid protein and viral genomic RNA. The canyon, the surface depression around each of the twelve icosahedral fivefold axes, is the most dominant structural feature among the picornaviruses. Although similarities in structure and sequence suggest that picornaviruses were derived from a common ancestor, they have evolved to use different cellular receptors for infection. For instance, the human rhinovirus major group viruses use intercellular adhesion molecule-1 (ICAM-1) while poliovirus uses the poliovirus receptor (PVR). These receptors belong to the immunoglobulin (Ig) superfamily and are composed of five and three Ig domains, respectively. Cryo-electron microscopy and image reconstruction analysis has revealed that while ICAM-1 and PVR bind to the canyon region of the virus, they bind in different orientations. Specifically, ICAM-1 stretches deeply into the canyon and interacts with the residues on the canyon floor. Importantly, the residues in the canyon rim region are hypervariable ...