Effective number of codons (abbreviated as ENC or Nc) is a measure to study the state of codon usage bias

Codon usage bias

Codon usage bias refers to differences in the frequency of occurrence of synonymous codons in coding DNA. A codon is a series of three nucleotides that encodes a specific amino acid residue in a polypeptide chain or for the termination of translation .There are 64 different codons but only 20...

es in genes and genomes. The way that ENC is computed has obvious similarities to the computation of effective population size

Effective population size

In population genetics, the concept of effective population size Ne was introduced by the American geneticist Sewall Wright, who wrote two landmark papers on it...

 in population genetics

Population genetics

Population genetics is the study of allele frequency distribution and change under the influence of the four main evolutionary processes: natural selection, genetic drift, mutation and gene flow. It also takes into account the factors of recombination, population subdivision and population...

. Although it is easy to compute ENC values, it has been shown that this measure is one of the best measures to show codon usage bias.

Since the original suggestion of the ENC, several investigators have tried to improve the method, but it seems that there is much room to improve this measure.