This is because identification of M in electron micrographs of virions is complicated by the presence of transmembrane regions of spikes and RNP, and, more importantly, intermolecular interactions could potentially affect coronavirus morphology, including M-M, M-E, M-N, M-S, M-RNA and N-RNA interactions (reviewed in (Hogue and Machamer, 2007) ), palmitin-mediated interactions involving S and E (Boscarino et al., 2008; Lopez et al., 2008; Thorp et al., 2006) , and envelope stretching caused by the packaged helical ribonucleoprotein (Barcena et al., 2009 ).