H is the attachment protein as for influenza. Fusion glycoprotein (F), which replaces the neuraminidase spike, enables the viral lipid bilayer to fuse directly with the plasma membrane within seconds and to release its RNA into the cytoplasm. F also causes syncytium (multinucleate giant cell) formation in cell culture and in vivo. The host antibody response to F is the most important at limiting entry and spread of virus from cell to cell.