Model of NS4B membrane association. Schematic of the proposed topology of NS4B relative to the ER membrane and reported functional properties (see introduction). Model for NS4B-CTD membrane association is discussed in this paper. Here we propose that positive residues (amino acids are indicated) are important for membrane targeting through the interaction with the negative head groups of phospholipids. Abbreviations: Nt, Amino terminus; Ct, Carboxyl terminus; bZIP, basic leucine zipper motif; TMx, transmembrane segment X.