A recombinantly expressed nucleotide triphosphate transporter efficiently imports the triphosphates of unnatural nucleotides into cells, and the endogenous cellular machinery incorporates those nucleotides into cellular nucleic acids. UBPs can therefore form within the cell's nucleic acids. Moreover, neither the presence of the unnatural triphosphates nor the replication of the UBP represents a significant growth burden. The UBP is not efficiently excised by nucleic acid repair pathways, and therefore can be retained as long as the unnatural triphosphates are available in the growth medium. Thus, the resulting cell is the first organism to stably propagate an expanded genetic alphabet.