Virus binding and penetration. As shown in Figure 9A , VSV initiates an infection by binding to a receptor such as phosphatidyl serine, a lipid component in the plasma membrane [19, 44] . After the binding step, the VSV particle is endocytosed via a clathrin-coated pit, and then penetrates intracellular vesicles such as endosomes by membrane fusion [19, [45] [46] [47] . The penetration leads to the release of the encapsidated negative-sense viral genome and virus proteins into the cytoplasm of the host cell. By assuming the binding step is ratedetermining [48] , we lump these early steps from the binding to the penetration into a first-order expression: