Both noroviruses and sapoviruses are genetically and antigenically highly diverse. The non-enveloped virus particles are 27-40 nm in diameter and have a single-stranded positive-sense RNA genome of 7.1-7.7 kb in length. In human noroviruses, this genome is divided into three open reading frames (ORF1-3) whereas sapoviruses consist of 2 ORFs (see Figure).

Based on phylogenetic clustering of complete VP1 amino acid sequences, noroviruses can now be classified into 10 established (GI - GX) and 2 tentative new norovirus genogroups, of which viruses from GI, GII, GIV, GVIII and GIX (formerly GII.15) infect humans [2]. Each genogroup can be further divided into genotypes (9 GI, 26 GII, 2 GIV, 1 GVIII, and 1 GIX). In addition, for GI and GII at least 14 GI polymerase (GI.P) types and 37 GII.P types have been described based on clustering of nucleotide sequences of the polymerase region [2].

Most laboratories amplify relative short sequences spanning the 3’-end of ORF1 (polymerase region) and 5’-end of ORF2 (major capsid protein gene) to genotype GI and GII noroviruses. These sequences can be generated in separate RT-PCR reactions or in a single, genogroup-specific, RT-PCR assay [3].

This human calicivirus typing tool [4] uses a BLAST-like algorithm to type both the polymerase region (P-type) and the capsid (genotype) of GI, GII, GIV, GVIII, and GIX (formerly GII.15) noroviruses as well as capsid genotyping of GI, GII, GIV and GV sapoviruses [5] against a database of norovirus and sapovirus reference sequences. Genotypes are assigned following classification criteria agreed upon by the Norovirus Classification Working Group [2].

Reference sequences

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