English: General introduction to MERS-CoV: model structure, life cycle and genomic composition. (A) Cartoon model structure of MERS-CoV. (B) Membrane fusion mechanism for MERS-CoV spike glycoprotein. Binding between RBD and the cell receptor (DPP4) triggers the conformational change of S glycoprotein to form a pre-hairpin intermediate of S2, in which the hydrophobic HR1 is exposed and the fusion peptide inserts into the target cell membrane. This transient S2 intermediate then refolds with HR2 into a stabilized trimer of hairpins, also called six-helix bundle structure (6-HB), bringing the target cell membrane into close proximity of the viral envelope and resulting in the completion of the fusion process. (C) Genomic composition of MERS-CoV. Each coloured box (length in scale) represents one open reading frame in the genomic RNA. The schematic for spike glycoprotein was also shown with labelled domain names and residue numbers. ORF (open reading frame), DPP4 (dipeptidyl peptidase 4), RBD (receptor-binding domain), NTD (N-terminal domain), CTD (C-terminal domain), FP (fusion peptide), and HR1-2 (heptad repeats 1-2).