Knight Z.A. et al. Cell 151, 1126–1137 (2012).

Behaviors are controlled by the activation of discrete populations of neurons in the brain. To understand this process better, it is necessary to define the molecular identity of those functional populations of neurons. In recently activated neurons, the ribosomal protein S6 gets phosphorylated, so Knight et al. decided to use this as a tag. The researchers exposed mice to different types of stimuli and then immunoprecipitated phosphorylated ribosomes from the brain homogenates to profile the associated messenger RNAs. By comparing the abundance of each transcript in the phospho-S6 immunoprecipitate to its abundance in the tissue as a whole, they obtained lists of genes uniquely expressed by populations of neurons that responded to specific stimuli. The molecular identification of activated cells may also serve to identify new markers that define these subpopulations of interest.