The genetic code is degenerate, i.e. some amino acids are coded for by more than one codon. Although coding for the same amino acid, synonymous codons are not equally used in different genomes, and even in a single genome the synonymous codon usage can vary widely among genes, or even along the gene sequences. With the recent advent of full genome sequences we are starting to unravel the reasons for these deviations. In this review we will cover some of the proposed factors that might cause different codon usage bias and the selective forces influencing it.
codon usage; codon bias; translation; expression level; selection