Much of the excitement around gene editing is fueled by its potential to treat or prevent human diseases. There are thousands of genetic disorders that can be passed on from one generation to the next; many are serious and debilitating. Genes are the biological templates the body uses to make the structural proteins and enzymes needed to build and maintain tissues and organs. They are made up of strands of genetic code, denoted by the letters G, C, T and A. Humans have about 20,000 genes bundled into 23 pairs of chromosomes all coiled up in the nucleus of nearly every cell in the body.