The virus buds through an area of cell membrane which has already been perforated by several hundred viral glycoprotein (G protein) spikes. This becomes its envelope. Matrix protein (M) acts as a scaffolding between the nucleocapsid and envelope.

The envelope includes the host cell lipid bilayer and is therefore more pleomorphic and fragile than the capsid. It is also more easily lysed by detergents, disinfectants or damaged by the outside environment.

The animal viruses with helical symmetry all have envelopes.

A minority of the icosahedral animal viruses have envelopes, but most do not.