Reply to Hettinger: Hydrophobic unnatural base pairs and the expansion of the genetic alphabet

We have recently reported the successful creation of a semisynthetic organism that has an expanded genetic alphabet by virtue of the retention on a plasmid of either the dNaM-d5SICS or dNaM-dTPT3 unnatural base pair (UBP), the latter of which can be retained at natural-like levels with the use of a Cas9 editing system (1, 2). Despite these demonstrations that a cell can retain a UBP in its DNA, Hettinger (3) now concludes that such hydrophobic UBPs are not suitable for the expansion of the genetic alphabet. Hettinger cites Hirao and Kimoto (4), who note that self-pairing may be a …