6CS2:
reference: Stabilized coronavirus spikes are resistant to conformational changes induced by receptor recognition or proteolysis., Kirchdoerfer RN, Wang N, Pallesen J, Wrapp D, Turner HL, Cottrell CA, Corbett KS, Graham BS, McLellan JS, Ward AB, Sci Rep. 2018 Oct 24;8(1):15701. doi: 10.1038/s41598-018-34171-7. PMID: 30356097

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