Natural selection and mutation pressure has been considered to be two key factors which have effect on codon usage patterns of organisms [19] . In order to explore whether determinative factors for codon usage mutation in PCV is mutation pressure or natural selection, we compared correlation between A 3 %, U 3 %, G 3 %, C 3 %, GC 3 % and U 3 %, G 3 %, C 3 %, GC 3 % with correlation analysis ( Table 3 ). The analysis results show, except GC %, GC3% has marked correlation with A%, U%, G% and C%. This indicates that GC 3 % can reflect interaction between natural selection and mutation pressure to some extent.