In addition, the correlation between f'1,f'2 value and A %, U%, G%,C%, GC%, A 3 %, U 3 %, G 3 %, C 3 %, GC 3 % was analyzed ( Table 4 ). The results showed that significant correlations exist between synonymous codon usage pattern and nucleotide composition in PCV. This result further verifies the conclusion that during the shaping of synonymous codon usage pattern of PCV, Composition constraints play and very important role. 