S trimers are recruited to the assembling virion through interaction with M protein (reviewed in Ujike and Taguchi, 2015). Multiple regions of M contribute to the recruitment of S, with a single tyrosine residue in the C-terminal domain of M playing a critical role (McBride et al, 2010a; Hsieh et al, 2008). Interaction with M is aided by a dibasic motif in the C-terminus of S, which promotes retrieval of the spike protein from the cell surface by binding the COPI coat (McBride et al, 2007; Ujike et al, 2016). Palmitoylation of the C-terminus of S appears dispensible for the interaction with M in SARS-COV-1, unlike the case in other coronaviruses (Ujike et al, 2012; McBride 2010b; reviewed in Ujike and Taguchi, 2015). Size estimates and modelling suggest the mature virion has approximately 300 S trimers (Neuman et al, 2006; reviewed in Chang et al, 2014).