In two-dimensional electron gases, the cyclotron effective mass is defined only for one magnetic field direction (perpendicular) and the out-of-plane wavevector drops out. The cyclotron effective mass therefore is only a function of energy, and it turns out to be exactly related to the density of states at that energy via the relation g ( E ) = g v m ∗ ∗ π π ℏ ℏ 2 , where g v is the valley degeneracy. Such a simple relationship does not apply in three-dimensional materials.