The core structure consists  of a five-stranded antiparallel β-sheet and several short connecting α-helices. The extended loop lies on one edge of the core structure    and forms a gently concave surface with two ridges on both sides and a two-stranded antiparallel β-sheet   sitting in the middle. Because this extended loop makes all contacts with ACE2, it has been termed receptor-binding motif (RBM).