C virus, or John Cunningham virus (JCV), is a member of the Polyomaviridae family and the causative agent of Progressive Multifocal Leukoencephalopathy (PML), a life-threatening viral infection of the brain. The viral capsid is about 40 nm in diameter and is formed by 72 pentamers of the VP1 protein. Each pentamer is associated with one molecule of either VP2 or VP3, the 2 minor capsid proteins. Only VP1 is exposed on the surface of the virus and it is therefore the protein responsible for the receptor binding.