Scheme 1. Schematic illustration of a mRNA-triggered primer exchange reaction (PER)-based DNA machine. In step 1, a dumbbell shaped hairpin with two loops (PER hairpin) was first designed. One loop (domain c, blue) was programmed to be complementary to the sequences of the survivin mRNA, and could hybridize with the survivin mRNA and expose the complementary domain of the primer a* (green). In step 2, primer a with a length of 8 nucleotides subsequently bound with domain a* and was extended using domain b* as the template such as in the PCR with the aid of Klenow (exo-)fragment polymerase to append a nascent single-stranded sequence b (termed the copied b domain, cb) to the 3′-end of the short primer until the strand displacing elongation reaction was halted at the stop sequence, two pairs of synthetic nucleotides, iso-dG (2′-O-methyl G) and iso-dC (2′-O-methyl C), on the PER hairpin. In step 3, the b domain on the hairpin was then able to compete with the copied b domain (cb) via the random walk process of three-way branch migration, and the b domain on the hairpin again hybridized with domain b*. In step 4, the copied b domain was displaced and a part of the 8 nucleotide long primer also was away from the PER hairpin due to the lower hybridization temperature. The extended primer was spontaneously dissociated from the PER hairpin. The PER hairpin became free and bound with another primer to induce the next cycle of the primer exchange reaction.