Panel A shows a thin-section electorn-microscopical view of viral nucleocapsids aligned along the membrane of the rough endoplasmic reticulum (arrow) as particles bud into the cisternea. Enveloped virions have surface projections (arrowhead) and an electron-lucent center. Directly under the viral envelope lies a characteristic ring formed by the helical nucleocapsid, often seen in cross-section. Negative stain electron microscopy (Panel B) shows a stain-penetrated coronavirus particle with an internal helical nucleocapsid-like structure and club-shaped surface projections surrounding the periphery of the particle, a finding typical of coronaviruses (methylamine tungstate stain). The bars represent 100 nm.