A semi-working copy of the page is at http://mapmaker2.millersville.edu/pamaps/PA_CountyHistories/. The goal is to have the user click on the tabs to the left of the map. The right two tabs include nested unordered lists, and each list item is a <button>. In the function called by the button click, the current feature layer is removed and the new feature layer added. At more detailed levels of the nested lists the added feature layer should have a subset of the features visible, by virtue of a setDefinitionExpression method. I believe that the definition expression is not working because the previous definition expression is not being cleared before the new one is added. The API Reference is very vague about that behavior, and the only methods there are "get..." and set..." . There is no "clear" or "remove." Any ideas?