Understanding which of the many classes of neuron are involved in particular mammalian brain functions is a major challenge in neuroscience. This study shows that the ribosomal protein S6 is phosphorylated in mouse neurons in response to a wide range of stimuli. By exposing mice to a stimulus, such as fasting or altered salt balance, and then immunoprecipitating phosphorylated S6 from brain homogenates, the authors were able to detect associated mRNAs that are expressed in response to the stimulus. Some of these mRNAs are cell-type-specific, allowing identification of the activated neuronal types.