Within the species Rotavirus A there are different strains, called serotypes. As with influenza virus, a dual classification system is used based on two proteins on the surface of the virus. The glycoprotein VP7 defines the G serotypes and the protease-sensitive protein VP4 defines P serotypes. Because the two genes that determine G-types and P-types can be passed on separately to progeny viruses, different combinations are found. A whole genome genotyping system has been established for Rotavirus A, which has been used to determine the origin of atypical strains. The prevalence of the individual G-types and P-types varies between, and within, countries and years. There are at least 36 G types and 51 P types but in infections of humans only a few combinations of G and P types predominate. They are G1P[8], G2P[4], G3P[8], G4P[8], G9P[8] and G12P[8].
