Sketch map of the transmembrane topology of viroporins. The four types of viroporins based on their transmembrane topologies. (A) and (B) represent type IA and type IB viroporins, respectively. These viroporins possess only one transmembrane domain with inverse orientations. (C) and (D) represent type IIA and type IIB viroporins, respectively. These viroporins possess two transmembrane domains with opposite orientations. Adapted from .