Scaffold protein-mediated ion channel clustering at unique membrane sites is important for electrical signaling. Yet, the mechanism(s) by which scaffold protein-ion channel interactions lead to channel clustering or how cluster ion channel density is regulated is mostly not known. the voltage-activated potassium channel (Kv) represents an excellent model to address these questions as the mechanism underlying its interaction with the post-synaptic density 95 (PSD-95) scaffold protein is known to be controlled by the length of the extended 'ball and chain' sequence comprising the C-terminal channel region. Here, using sub-diffraction high-resolution imaging microscopy, we show that Kv channel 'chain' length regulates Kv channel density with a 'bell'-shaped dependence, reflecting a balance between thermodynamic considerations controlling 'chain' recruitment by PSD-95 and steric hindrance due to the spatial proximity of multiple channel molecules. our results thus reveal an entropy-based mode of channel cluster density regulation that mirrors the entropy-based regulation of the Kv channel-PSD-95 interaction. The implications of these findings for electrical signaling are discussed. Action potential generation, propagation and the evoked synaptic potential all rely on precisely timed events associated with activation and inactivation gating transitions of voltage-dependent Na + and K + channels, clustered in multiple copies at unique membrane sites, such as the initial segment of an axon, nodes of Ranvier, pre-synaptic terminals or at the post-synaptic density (PSD) 1-3. Changes in either ionic current shape or density, reflecting changes in temporal and spatial dimensions, respectively, affect action potential shape and frequency and may lead a neuron to change its mode of firing 4-9. Despite emerging evidence attesting to the importance of ion channel density for efficient electrical signaling 7 and information encoding 7-9 , little is currently known of the clustering process itself or its regulation. It is, however, clear that ion channel clustering is an active process, involving the interaction of a channel with a specific member of one of several scaffold protein families. For example, Nav channel clustering at nodes of Ranvier is mediated by interaction of the channel with the ankyrin G scaffold protein 10,11. Kv channel clustering at the PSD of excitatory synapses (of Drosophila melanogaster but not of mammalians), on the other hand, is mediated by binding to the PSD-95 synapse-associated scaffold protein 12-15. Yet, the mechanism by which these elementary binding events lead to the clustering of ion channel molecules in a restricted area of the membrane remains unclear. Furthermore, it is also generally not known if and how ion channel membrane density is regulated in the spatial and/or temporal dimensions. In the absence of a molecular mechanism describing the channel protein-scaffold protein interaction, bridging this molecularcellular gap to understand ion channel clustering has proven challenging. The pr...