A, schematic of the G4 DNA and the magnetic tweezers measurement. The G4-forming ssDNA is sandwiched between two dsDNA handles (489 and 601 bp). B, typical force-bead height curves of the Bcl2-2345 sequence in repeating stretching cycles in 100 mm KCl. The curves are shifted along the bead height axis for visual clarity. The G4 unfolding results in a sudden bead height jump (black arrows). C and D, unfolding force histograms for the Bcl2-2345 G4 in 100 mm KCl (C) and 150 mm KCl (D). E, unfolding step size histogram of the Bcl2-2345 G4 in 100 mm KCl (dotted line) and 150 mm KCl (solid line). Data were fitted by a single Gaussian equation. F, the time evolution of the folding probability pfold(t) of the Bcl2-2345 sequence in 100 mm KCl (open circles) and in 150 mm KCl (solid circles).