Extending Marionette.Region.prototype is a bad idea. You should define your own custom region class and use it with only those regions that you want
Example:
var HeaderRegion = Backbone.Marionette.Region.extend({
open: function(view){
this.$el.hide();
this.$el.html(view.el);
this.$el.slideToggle("slow");
}
});
MyApp.addRegions({
headerRegion: {
selector: "#header",
regionType: HeaderRegion
}
});
For more information read Marionette.Region documentation