This is a perennially recurring article. Seems like every few years someone is adding base pairs to DNA. The first one I can recall used the Greek letters kap

This is a perennially recurring article. Seems like every few years someone is adding base pairs to DNA. The first one I can recall used the Greek letters kappa and chi for the novel base pair. I thought it was cool and exciting at the time, which was probably around 1992. They keep coming up with new ones, and reviving the "6 letter DNA alphabet" articles. 2-amino-8-(2-thienyl)purine and pyridine-2-one 7-(2-thienyl)imidazo[4,5-b]pyridine and pyrrole-2-carbaldehyde 7-(2-thienyl)imidazo[4,5-b]pyridine and 4-[3-(6-aminohexanamido)-1-propynyl]-2-nitropyrrole 2-(2-Deoxy-β-D-erythro-pentofuranosyl)-6-methyl-1(2H)-isoquinolinethione and (1R)-1,4-Anhydro-2-deoxy-1-(3-methoxy-2-naphthyl)-D-erythro-pentitol These all work fine when copying DNA sequences using existing cellular mechanisms and PCR. As far as I know, it remains to be seen whether they can encode for proteins. RNA transcription is complicated by "wobble pairs" with uracil, inosine, and uridine variants, occurring in RNA, with the four bases present in DNA, and with each other. There isn't a 1-to-1 correspondence from DNA base to RNA base. It may be that our DNA uses only the four specific bases guanine, cytosine, adenine, and thymine because wobble pairings provided additional mutation resistance, or offered additional structural options for transcribed proteins.