Serotonin neurons innervate nearly the entire mammalian brain, yet how their projections are organized has remained unclear. Systematic whole-brain serotonergic axon collateralization mapping reveals five projectomic groups that preferentially target functionally related regions, with each projectomic group connecting to specific spatial transcriptomic and behavioral profiles.

Recent advances in RNA analysis have deepened our understanding of cellular states in biological tissues. However, a substantial gap remains in integrating RNA expression data with spatial context acr...