Phylogenic tree of GII-4 norovirus P domain sequences (without the hinge region) generated by using the PhyML maximum likelihood analysis. GII-4 sequences from 1987 to 2008 are shown in different colored clusters. Strains isolated in this study are as follows: 00Y96C (GU937455), 06Yb96C1 (GU937451), 06Yb96C2 (GU937451), 06Yb96C3 (GU937453), 00Y02C (GU937448), 03Y02C1 (GU937449), 03Y02C2 (GU937450), 06Y02C (GU937454), 02Y02C (GU937456), 08Y02C (GU937463), 03Y02C3 (GU937465), 05Y04C (GU937457), 06Y06aC1 (GU937458), 06Y06aC2 (GU937459), 06Y06aC3 (GU937460), 08Y06bC (GU937464), 08Y06bC1 (GU937462), and 08Y06bC2 (GU937461). These strains are highlighted by blue stars.