The results of our uniaxial stretching trials suggest that filaments and networks formed from mutant K14-R125P proteins do not differ from WT filaments and networks in their response to largescale uniaxial cell deformations. Specifically, we observed no evidence of keratin bundle or network rupture when the cells were subjected to uniaxial strains. These results undermine both the fragile filament and fragile network hypotheses described in the Introduction, although it is possible that even higher levels of uniaxial stretching could reveal differences between WT and mutant networks. It is important to note that in these experiments we were not able quantify the stress developed by the keratin filaments or network, only their morphological response to being deformed. To confirm the idea that mutant filaments do not differ in their material properties from WT filaments will require the application of emerging atomic force microscopy methods developed for the mechanical testing of single intermediate filaments. A study by Russell et al., is the only other to investigate the effects of mechanical stress on the morphology of WT and EBS mutant networks in keratinocytes. They subjected cells to a radial oscillatory strain regime, which resulted in breakdown of the keratin network and the appearance of aggregates in EBS mutant cells in as little as 15 min after initiation of the Desmethyl Erlotinib oscillating stretch regime. Taken together with our findings, these data suggest that mutant EBS cells respond differently to deformations that are fast, oscillating, and radial, versus those that are slow, acute and uniaxial. The differences in the results from our study and Russell et al. could be explained by two distinct mechanisms. One possibility is that mutant filaments are identical in their tensile mechanical behavior to WT filaments when they are loaded in a quasi-static mode to high strains, but they fall apart when they are loaded repeatedly to low strains at high strain rates. Another possibility is that the filaments that form in EBS mutant cells are mechanically identical to those in WT cells, but fast mechanical oscillations in mutant cells lead to a generalized cellular stress response that then leads to a breakdown of the keratin network via active cellular signaling mechanisms. The lack of evidence for a disruptive effect of the R125P mutation on the morphology of the keratin network in keratinocytes contradicts results by Ma et al., who report significant mechanical differences between gels formed from WT K14 and K14-R125C proteins in vitro. The authors attribute these differences to a bundling defect in mutant filaments. Observations of GFP labeled WT and mutant networks in live keratinocytes revealed no such bundling defect, although it is possible that defects may yet be revealed by electron microscopy. In a previous study, we demonstrated that NEB-1 cells expressing WT K14-GFP are able to survive dramatic uniaxial strains. In the current study, we found that cells expressing the dominant 1-Deacetylnimbolinin-B negative R125P mutation of the K14 gene associated with severe EBS are just as capable of surviving large-scale uniaxial strains. These results raise the important question of whether EBS mutant cells with intact keratin networks are as strong as WT cells. We showed that the two cell lines are capable of surviving the same magnitude of strain, but what if cell viability were measured as a function of stress ? Would the mutant cells be able to bear loads of the same magnitude as WT cells before failing? A recent study of EBS mutant and WT keratinocyte mechanics by Lulevich et al. provides some clues.