Relative synonymous codon usage (RSCU) is defined as the ratio of the observed frequency of codons to the expected frequency given that all the synonymous codons for the same amino acids are used equally. RSCU values have no relation to the amino acids usage and the abundance ratio of synonymous codons, which can directly reflect the bias of synonymous codon usage (Sharp and Li, 1986) . The codon adaptation index (CAI) was used to estimate the extent of bias towards codons that were known to be preferred in highly expressed genes (Sharp and Li, 1987) . It is now proved that CAI values mostly approach the theoretical values to reflect the expression level of a gene. Thus it has been widely utilized to measure the gene expression level (Naya et al., 2001; Gupta et al., 2004) . A CAI value ranges from 0 to 1.0, and a higher value indicates a stronger codon usage bias and a higher expression level.