Doubly-expanded (or naphtho-homologated) nucleobases incorporate a naphthalene spacer instead of a benzene ring, widening the base twice as much with its two-ringed structure. These structures (known as xxDNA and yyDNA) are 4.8Å wider than natural bases and were once again created as a result of Leonard's research on expanded adenine in ATP-dependent enzymes in 1984. No literature was published on these doubly-expanded bases for nearly three decades until 2013 when the first xxG was produced by Sharma, Lait, and Wetmore and incorporated along with xxA into a natural helix. Although very little research has been performed on xxDNA, xx-purine neighbours have already been shown to increase intrastrand stacking energy by up to 119% (as opposed to 62% in x-purines). xx-purine and pyrimidine interactions show an overall decrease in stacking energies, but the overall stability of duplexes including pyrimidines and xx-purines increases by 22%, more than twofold that of pyrimidines and x-purines.