SHANK3 is a postsynaptic protein that plays a role in synaptic development and function. In humans, mutations in SHANK3 have been associated with autism spectrum disorders (ASDs). SFARI Investigator Guoping Feng and his colleagues now show, using a novel SHANK3 conditional knock-in mouse model, that re-expression of the SHANK3 gene in adult mice leads to improvements in synaptic protein composition, spine density and neural function in the striatum. Select behavioral abnormalities, including deficits in social interaction and repetitive grooming, could be rescued in the adult.  By contrast, anxiety-like behaviors and motor coordination deficits are irreversible in the adult but can be improved by early postnatal intervention. Combined with previous studies showing adult rescue of select cellular and behavioral phenotypes in other genetic mouse models of ASDs, these data provide hope that a certain degree of improvement may be achievable in individuals with ASDs, even after early developmental milestones are completed.