GC content is the frequency of G+C in a coding gene. GC1, GC2 and GC3 contents are the frequencies of G+C at the first, second and third positions of codons, respectively. A3, T3, G3 and C3 contents are the frequencies of A, T, G and C at the synonymous third position of codons, respectively. Effective number of codons (N c ), ranging from 20 to 61, is generally used to measure the bias of synonymous codons. When N c value approaches 20, only one codon is used with extreme bias for one amino acid and, if the value is up to 61, the anonymous codons are used equally with no bias (Wright, 1990) .