Flaviviruses bud into the endoplasmic reticulum (ER) and are transported as virus particles through the secretory pathway and released by exocytosis [4] . Given the low pH that is present in the Golgi complex and trans-Golgi network (TGN) [31] , how do flaviviruses avoid inactivation during their transport? The particles are assembled in the ER as immature non-infectious viruses containing heterodimers of the precursor membrane protein (prM) and E protein [4, 26, 32] . Subsequent exposure to low pH in the secretory pathway triggers a dramatic rearrangement to E homodimers and makes the prM protein accessible to furin cleavage [33, 34] . Processing of prM by cellular furin results in mature infectious virus in which E homodimers are poised to mediate fusion [33] . Important recent studies describe the structure of pr peptide in complex with E, and indicate that processed pr remains associated with the virus at low pH and can inhibit virus-membrane interaction [34, 35, 36] . Thus, pr on the virus could protect E protein from low pH in the secretory pathway.