To confirm that the folding characteristics of HP35 and HP35st are preserved when the peptides are inserted between two fluorophores, we purified HP35-based tension sensor modules from cells and performed single-molecule force spectroscopy calibration. Those experiments corroborated the previously observed folding/unfolding kinetics of the isolated peptides and revealed the mid-force transition to be at about 7.4 pN for HP35 and 10.6 pN for HP35st (Austen et al., 2015). The observed folding transitions were well described by a theoretical model assuming HP35/HP35st to be either in a folded, half-folded/half-unfolded or unfolded state, and the resulting probabilities for HP35/HP35st to be in any of these conformations at a given force were then used to calculate the sensor module force-FRET responses (Fig. 3). Furthermore, force spectroscopy experiments showed that the employed fluorophores (YPet and mCherry) do not unfold at low pN forces and even remained stably folded when exposed to a constant force of 24 pN for more than 5 min.