The Codon Adaptation Index (CAI)  is the most widespread technique for analyzing codon usage bias. As opposed to other measures of codon usage bias, such as the effective number of codons (Nc), which measure deviation from a uniform bias (null hypothesis), CAI measures the deviation of a given protein coding gene sequence with respect to a reference set of genes. CAI is used as a quantitative method of predicting the level of expression of a gene based on its codon sequence.