A new examination of the way different tissues read information from genes has revealed that the brain and testes appear to be extraordinarily open to the use of many different kinds of code to produce a given protein. In fact, the testes of both fruit flies and humans seem to be enriched in protein products of these rarely-used pieces of genetic code. The researchers say the use of rare pieces of code may be another layer of control in the genome that could be essential to fertility and evolutionary innovation. A decade after solving the structure of DNA as a double helix of the bases A,C, T, and G, Francis Crick went on to decode the intermediate step by which three of these letters are translated into a “codon,” the recipe for a single amino acid, the building block of protein. What was striking at the time and still somewhat puzzling is that this layer of life’s code used 61 different three-letter codons to produce just 20 amino acids, meaning many codons were being used to describe the same thing.