 Oligomers of these proteins, most often tetramers, form ion channels or pores of usually weak ion selectivity that permit diffusion of ions across the cell membrane.The molecular architecture of the pore, its degree of selectivity, the extent to which it incorporates lipids from the surrounding membrane, and the presence of portions of the protein that extend beyond the membrane all vary among viroporins and indicate that these proteins have a diverse array of functional roles.A proposed classification scheme sorts viroporins into four classes based on their topology and orientation in the membrane.