In canine parvovirus (CPV) and feline panleukopenia virus (FPV), large insertions between strands of the β-barrel form most of the capsid surface and create small, protruding "spikes" around the icosahedral threefold axes, which are involved in host recognition and antigenicity. Whereas both viruses can utilize the feline transferrin receptor (TfR) for attachment and infection, CPV gained the ability to bind canine TfR and to infect canine cells and dogs. Residues 93 and 323 within the major capsid protein, located in the vicinity of the threefold spikes, allow the CPV capsids to bind canine TfR.