The genetic code of life is made up of building blocks called nucleotides. Since the discovery of the code in the 1960's, one mystery has been that there is more than one version of the code to build the same protein (proteins are the product of the code). The code is made up of triplet nucleotides, known as codons. Some codons are common while others are rare (in the above figure, the AAG codon is more common in the genetic code of the fly Drosophila than the AAA codon. We've found that different tissues interpret these different versions of the code in distinct ways, and that these differences underly why, for example, a brain is a brain and not an intestine. Our work is impacting understanding of fundamental differences between organs, and is especially important for male fertility and the nervous system.