The codon bias index reflects the composition of a gene with high expression of superior codons. For the gene of the target host itself, the index has a good correlation with the ENC value, and it can more clearly reflect the possible expression of foreign genes in the target host, so it has been widely used.
Where Npf r is the total number of occurrences of preferred codons, Nra n is the expected number of the preferred codons if all synonymous codons were used equally, and Ntot is the total number of the 17 amino acids encoded by the preferred codons. A strongly expressed gene has a higher value of CBI and a more biased codon usage than a weakly expressed gene.
It measures the extent to which a gene uses a subset of optimal codons.
When CBI is equal to 1.0, it indicates a gene with extreme codon preference. On the contrary, when CBI is equal to 0.0, it means that the gene has random codon usage.
Codon Selection in Yeast