Hepatitis E virus (HEV) is an important etiological agent of epidemic and sporadic hepatitis, which is endemic to the Indian subcontinent and prevalent in most of the developing parts of the world. The infection is often associated with acute liver failure and high mortality, particularly in pregnant women. In order to develop methods of intervention, it is essential to understand the biology of the virus. This is particularly important as no reliable in vitro culture system is available. We have constructed a cDNA clone encompassing the complete HEV genome from independently characterized subgenomic fragments of an Indian epidemic isolate. Transfection studies were carried out with HepG2 cells using in vitro-transcribed RNA from this full-length HEV cDNA clone. The presence of negative-sense RNA, indicative of viral replication, was demonstrated in the transfected cells by strand-specific reverse transcription-PCR and slot blot hybridization. The viral proteins pORF2 and pORF3 and processed components of the pORF1 polyprotein (putative methyltransferase, helicase, and RNA-dependent RNA polymerase) were identified in the transfected cells by metabolic pulse-labeling with Hepatitis E virus (HEV) is established as an etiological agent of the epidemic and sporadic forms of waterborne hepatitis (6, 25). The first well-characterized HEV epidemic was reported in Delhi, India, in 1955 (35). Several other epidemics have since been described in developing countries on nearly every continent (19,21). HEV has a positive-strand polyadenylated RNA genome ϳ7.2 kb in length (27) containing three open reading frames (ORFs). Nonstructural ORF1 (5Ј end) codes for a polyprotein of 1,693 amino acids (pORF1) and contain domains homologous to a viral methyltransferase, a papainlike cysteine protease, an RNA helicase, and an RNAdependent RNA polymerase (RdRp). The second ORF (3Ј end) codes for the major viral capsid protein of 660 amino acid (pORF2), while the third and smallest ORF (ORF3) codes for a 123-amino-acid-long polypeptide (pORF3) whose function is unknown (32). Apart from its coding region, the viral genome has 27-and 68-nucleotide (nt)-long noncoding regions at its 5Ј and 3Ј ends, respectively (32). The genome sequences of HEV have been reported from different geographical isolates and show a high degree of homology at both the nucleotide and amino acid levels (3,4,12,32,33). Expression of structural proteins pORF2 and pORF3 in prokaryotic and eukaryotic systems has been reported by different investigators (7,11,14,23,29). Earlier, we have expressed and characterized pORF2 and pORF3 in animal cells. pORF2 is an 88-kDa glycoprotein which is expressed intracellularly, as well as on the cell surface (14, 37). The ORF3 protein (pORF3) is a 13.5-kDa phosphoprotein which is phosphorylated by the cellular mitogen-activated protein kinase and associates with the cytoskeleton (36). We have also recently expressed the ORF1 polyprotein in both prokaryotic and eukaryotic systems (2).In the absence of a reliable in vitro culture system...