they began their experiment by constructing a pair of new bases, dubbed S and Y. These combine in the same way as A and T or G and C, so they can fit into DNA molecules without disrupting them.
M. Liffiton, B. Sheese, J. Savelka, and P. Denny. Proceedings of the 23rd Koli Calling International Conference on Computing Education Research, page 1–11. New York, NY, USA, Association for Computing Machinery, (Feb 6, 2024)