The S2 fusion machinery is the best resolved part of the map, whereas the SA and SB domains are less well resolved, presumably because of conformational heterogenity. The atomic model comprises  residues 27-1147, with internal breaks corresponding  to flexible regions (including part of the RBM), and lacks the C-terminal most segment that is not visible in the map, as is the case for all S structures determined to date. Overall, the SARS-CoV-2 S ectodomain is a 160-A-long trimer with a triangular cross-section, resembling the closely related SARS-CoV S structure.