DNA is the genetic material of all organisms on Earth; it is a self-replicating protein material made up of a string of amino acids that combine to form genes, which carry the ‘instructions’ for plants, animals and humans to grow, develop, function or reproduce. Now, a team of researchers in California have begun challenging the rules of these genes, re-writing our definition of what it means to be ‘alive’ by inserting ‘man-made’ genetic letters into a DNA code. This process, or ‘semisynthetic life’, as it has become known, holds the potential to fundamentally change how we approach the fields of biology and chemistry, and therefore medicine, in a fundamental way.