S protein from a yet-unknown virus was passed into the ancestor and gave rise to CCoV, whose S protein was again recombined into FCoV I to form FCoV II. CCoV gradually evolved into TGEV. FCoV type II Virus fusion Coronaviruses are covered with several types of "S proteins" (or E2) forming a crown of protein spikes on the surface of the virus. Coronaviruses take their name from the observation of this crown by electron microscopy. These spikes of Cov (group 1 and serotype II) are responsible for the infection power of the virus by binding the virus particle to a membrane receptor of the host cell—the Feline Amino peptidase N (fAPN). The viral receptor: aminopeptidase N (APN) fAPN (feline), hAPN (human) and pAPN (porcine) differ in some areas of N-glycosylation. All strains of the coronavirus study group 1 (feline, porcine and human) can bind to the feline aminopeptidase N fapn but the human coronavirus can bind to the human APN (HAPN) but not to the porcine type receptor (pAPN) and the pig coronavirus can bind to the porcine APN (pAPN) but not the human type receptor (hAPN). At the cellular level the glycosylation level of enterocytes APN is important for the binding of virus to the receptor. Viral spikes The FECV spikes have a high affinity for enterocytes fAPN, while the mutant FIPV spikes have a high affinity for the macrophages fAPN. During the viral replication cycle, spikes proteins mature in the host cell Golgi complex with a high mannose glycosylation. This spike manno-glycosylation stage is vital for the acquisition of coronavirus virility. Molecular model of FCoV type I The receptor In 2007, it was well established that serotype I did not work with the FCoV fAPN receptor. The FCoV type I receptor still is unknown. CoV receptor The human CoV SARS binds to the angiotensin-converting enzyme ACE II. The ACE II is also called L-SIGN (liver/lymph node-specific intracellular adhesion molecules-3 grabbing non-integrin). Coronaviruses bind to macrophages via the Dendritic Cell-Specific Intercellular adhesion molecule-3-Grabbing Non-integrin (DC-SIGN) which is a trans-membrane protein encoded in humans by the CD209 gene. ACE and DC-SIGN are two trans-membrane retrovirus receptors (mannose receptors) which can bind "the plant lectins C-type mannose binding domain". Aminopeptidase N has the same ability to interact with plant lectins C-type mannose-binding and also serves as a receptor for a retrovirus. Angiotensin-converting enzyme ACE, aminopeptidase A and aminopeptidase N have cascading