Fig. 4From: Vaccines platforms and COVID-19: what you need to knowSARS-CoV-2 Virion and Spike protein subunits. Subunit 1 S1-CTD (Citoplasmic Domain). S1 NTD (N-Terminal domain). S2 Subunit 2. Receptor Binding Domain [ RBD]. Angiotensin-converting enzyme 2Â (ACE2)Back to article page