Codon usage bias is the tendency of some organisms to use certain DNA triplets in preference to other, equally capable, triplets in building amino acids. Many of the 20 amino acids used by life can be built by different sequences of DNA; it isn't clear why some sequences are preferred.
Each of the 61 three-letter sequences of DNA known as triplets can code for an amino acid's production. As only 20 amino acids are used by life, some can be built by different pathways. Many organisms, such as baker's yeast, show a marked preference for using some triplets, or codons, for building amino acids over others. This bias is shown more often than chance alone would predict.