The extension of the unfolded single stranded HIV-1 hairpin, x SS was computed as x SS (F) = Áx + L HP for convenience, with F = (F 1 +F 2 )/2 the unfolding force and Áx the increase in extension ( Figure S2 ). The increase in contour length (expressed in number of nucleotides) was subsequently computed using the worm-like chain model for polymer elasticity assuming a stretching modulus of 1000 pN, a persistence length of 1 nm (43,44) and inter-phosphate distance of 0.59 nm. Alternatively, one may choose to do the computation taking into account F 1 and F 2 explicitly, where it is found that