Figure 1. Maximum-likelihood phylogeny of complete sequences of the major capsid protein of norovirus GII.4 variants. A) Nucleotide phylogenetic inference was computed using the Tamura-Nei model with gamma distribution of evolutionary rates among sites. A total of 1,617 positions were included in the final dataset. B) Amino acid phylogenetic inference was computed using the Jones-Taylor-Thornton model with gamma distribution of evolutionary rates among sites. A total of 536 positions were included in the final dataset. Best substitution models were selected using the lowest Bayesian Information Criterion scores. Magenta text indicates the 4 GII.4 Hong Kong sequences. Other GII.4 sequences used as references in the human calicivirus typing tool (https://norovirus.ng.philab.cdc.gov) were downloaded from GenBank. Sequence names are in the following format: GenBank accession no., virus strain name, 2-letter code of country/city of collection, year of collection. Bootstrap values >70% (of 100 iterations) are shown at nodes. Tree branches are drawn to scale; scale bars indicate number of substitutions per site. Trees are rooted to the oldest sequences collected from 1970s. GII.4 variants with pandemic spread are shown in bold text and annotated with the year of predominance (e.g., Sydney 2012); those without pandemic spread are labeled with variant names only (e.g., Osaka). AU, Australia; DE, Germany; HK, Hong Kong; IN, India; JP, Japan; NL, the Netherlands; PH, Philippines; UK, United Kingdom; US, United States.