Schematic representation of a feline coronavirus (FCoV) virion and genome organization. FCoV virions are 120–160 nm in diameter, roughly spherical, and enveloped. The envelope is decorated with 15- to 20-nm surface projections composed of trimers of the spike (S) glycoprotein. The nucleocapsid is helical, loosely wound, and composed of the viral genome and multiple copies of a single basic phosphoprotein species (N). The nucleocapsid helix is folded compactly. FCoV has a positive-sense single-stranded RNA genome with a linear topology, a 5′ cap, and a 3′ poly-A tail. The genome encodes the structural proteins S (spike), E (envelope), M (membrane), and N (nucleocapsid), as well as the nonstructural proteins 3a, 3b, 3c, 7a, and 7b. RNA replicase proteins are encoded by open reading frames 1a and 1b.