A general feature of temperature-induced reversible denaturation of small globular proteins is its all-or-none character.
This strong cooperativity leads to think that protein molecules, possessing only two accessible thermodynamic states, the
native and the denatured one, resemble ‘crystal molecules’ that melt at raising temperature. An analysis, grounded on mean
field theory, allows to conclude that the two-state transition is a first-order phase transition. The implication of this
conclusion are briefly discussed.