xDNA molecule. Computer artwork of a molecule of expanded deoxyribonucleic acid (xDNA). Normal DNA is composed of two strands twisted into a double helix. Each strand consists of a sugar-phosphate backbone attached to the nucleotide bases guanine, cytosine, thymine and adenine. xDNA, which was created by Professor Eric Kool and colleagues at Stanford University, USA, has an extra benzene ring added to adenine (xA) and thymine (xT). This makes the molecule wider and more heat resistant. It is also fluorescent, making it useful for medical biopsies. Synthetic forms of DNA, such as xDNA, could be used to engineer novel cell types.