Top 20 vertebrate species ranked in ascending order of squared Euclidean distance of RSCU to 2019-nCoV (A, D), SARS-CoV (B, E), and to MERS-CoV (C, F). The upper panels (A-C) are for all vertebrates with known codon usage in CoCoPUTS database, while the lower panel (D-F) is for the subset of species where more than 2000 CDSs are available for codon usage calculation.