 Class I viroporins possess a single transmembrane helix; in class IA the C-terminus is oriented into the cytosol and in class IB the N-terminus is so oriented.Class II viroporins possess a helix-turn-helix motif with both helices crossing the membrane; in class IIA both termini are oriented externally (extracellularly or toward the lumen of the endoplasmic reticulum) and in class IIB the termini are oriented toward the cytosol.Likely exceptions to this scheme exist, such as the rotavirus protein non-structural protein 4.