The S gene encodes a long ORF with three in-frame ‘start’ (ATG) codons that divide the gene into three sections, pre-S1, pre-S2 and S. HBV enters susceptible liver cells when the receptor-binding region of pre-S1 specifically interacts with the functional cellular receptor NTCP (sodium taurocholate cotransporting polypeptide), a multiple transmembrane transporter predominantly expressed in the liver; this results in liver infection and virus replication