The analysis  may explain why coronavirus S1-NTDs have evolved the ceiling-like structure  on top of the core, which is used to protect the sugar-binding site in coronavirus S1-NTDs from the host immune system. Subsequently, MHV S1-NTD took advantage of the ceiling-like structure and evolved CEACAM1-binding RBM on the outer surface of this ceiling like structure. In this sense, the evolution of CEACAM1-binding RBM in MHV S1-NTD might be an indirect outcome of the efforts of coronaviruses to battle the host immune attactks.