Recent results on magnetoresistance in a two-dimensional electron gas under crossed magnetic and microwave fields show a new class of oscillations, suggesting a new kind of zero-resistance states. We consider the problem from the point of view of the electronic structure dressed by photons due to a in-plane linearly polarized ac field. The dressed electronic structure includes opening of radiation induced gaps that have been overlooked so far and could play a role in the recently observed oscillations in the transverse magnetoresistance.