We define a homogeneous parabolic De Giorgi classes of order 2 which suits a mixed type class of evolution equations whose simplest example is mðxÞ qu qt À Du ¼ 0 where m can be positive, null and negative, so that elliptic-parabolic and forward-backward parabolic equations are included. For functions belonging to this class we prove local boundedness and show a Harnack inequality which, as by-products, gives Hö lder-continuity, in particular in the interface I where m change sign, and a maximum principle.