  We discuss the $\sqrt{s}$-dependence of the $K^+/\pi^+$ ratio in heavy-ion
collisions (the "horn" effect) within a Mott-Anderson localization model for
chemical freeze-out. The different response of pion and kaon radii to the hot
and dense hadronic medium results in different freeze-out conditions. We
demonstrate within a simple model that this circumstance enhances the "horn"
effect relative to statistical models with universal chemical freeze-out.
