Synthetic biologist Floyd Romesberg is developing a third, artificial base pair for DNA

[MTHead]

And in an old favorite of mine, Roger Zelazny’s Doorways In The Sand. Needless to say, I liked it better than this reviewer did:
https://www.tor.com/2009/08/27/beautiful-poetic-and-experimental-roger-zelaznys-doorways-in-the-sand/