Among enveloped viruses that achieve maturation through a nucleocapsid-independent assembly mechanism, HBV has developed the most active budding process that is known. It is carried out by the S-HBsAg envelope protein, and it leads to the formation of a large excess of empty subviral particles over mature virions. For that reason, HBV appears to be the best-suited virus to supply HDV with transport vesicles. For cell egress, the HDV RNP must be embedded in the HBV envelope proteins, and it is likely mediated by a specific interaction between L-HDAg and S-HBsAg. Recent experiments based on genetic analysis have identified a short sequence at the carboxyl terminus of S-HBsAg as a candidate for interaction with L-HDAg. At viral entry, assuming that HBV and HDV use a single primary receptor, the recent advance made in the identification of a HBV receptor binding domain in the L-HBsAg protein is likely to be relevant for HDV. Overall, there are still many important and interesting questions about the HBV envelope protein functions in the HDV life cycle, but to better understand morphogenesis and infectivity of HDV, we would need to obtain ultrastructural data that reveal both the architecture of the envelope and the precise organization of the RNP.

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