Abstract: The immediate early gene, Arc, is a pivotal regulator of synaptic plasticity, memory, and cognitive flexibility. But what is Arc protein? How does it work? Inside the neuron, Arc is a protein interaction hub and dynamic regulator of intracellular signaling in synaptic plasticity. In remarkable contrast, Arc can also self-assemble into retrovirus-like capsids that are released in extracellular vesicles and capable of intercellular transfer of RNA. Elucidation of the molecular basis of Arc hub and capsid functions, and the relationship between them, is vital for progress. The talk will present recent findings on Arc structure-function and regulation of oligomerization that are giving insight into the molecular physiology of Arc. As the functional states of Arc are revealed, a major goal is to selectively manipulate specific forms of Arc-dependent plasticity and intercellular communication involved in brain function and dysfunction.