Envelope protein required for virus entry into host cell and for cell-cell fusion (syncytium formation). Component of the intracellular mature virion (IMV) membrane. Contains two intramolecular disulfide bonds. They are created by the viral disulfide bond formation pathway, a poxvirus-specific pathway that operates on the cytoplasmic side of the MV membranes (By similarity). Belongs to the poxviridae A28 protein family.