Two possible denaturation mechanisms depending on the rate at which native contacts break can be devised for a given quasisigmoidal denaturation profile. In a slow-fast mode denaturation (a), the rate at which native contacts are broken is essentially constant at the beginning and then a higher increase of the rate is observed at the end. For the fast-slow mode denaturation (b), a small amount of denaturant induces a higher rate of denaturation in the beginning. Then, the process is essentially constant up to the critical concentration.