There are 64 different codons that encode 20 amino acids and three stop codons, meaning that the same amino acid can be encoded by more than one codon. Although the genetic code is universal, many different organisms actually prefer certain codons over others for certain amino acids. This is termed codon usage bias. In fact, some species are known to avoid certain codons altogether.
So what does this mean for a molecular biologist who wants to express genes from one organism in another? Let’s take a look at codon usage and when you might want to optimize codons for expression in a particular organism.