The cloning and sequencing of the hepatitis D virus (HDV) genome in 1986 established the peculiar features that make HDV the only genus of the Deltaviridae family ( 1 ). HDV is a defective negative-strand RNA virus that requires concurrent infection with the hepatitis B virus (HBV) to complete its replicative cycle. The 36-nm virion consists of the delta antigen (HDV Ag) and the RNA genome of 1.6 kb within an envelop of surface antigen (HBsAg), provided by the HBV infecting the host ( 2 , Fig. 1 ). The structure and biological properties and replication mechanism of HDV are similar to those of plant RNA viruses such as the viroids and satellite RNAs ( 3 ). Fig. 1.? Schematic virion of HDV.