Inhibitory interneurons in the cortex differentially express several types of markers: somatostain, cholecystokinin, neuropeptide Y, vasointestinal peptide, calbindin, calretinin, and parvalbumin. By measuring the distribution of cells expressing these markers in tissue, we can both constrain and validate the composition of neurons used in building brain microcircuits. Slices are also stained for NeuN, a marker of neurons, and GABA, which permits identification of inhibitory interneurons. Several samples per marker are needed to obtain an accurate measurement. Molecular properties of neurons in the rat somatosensory cortex.