Layer 4 neurons express the NR2C subunit of NMDAR. A, Horizontal section from the S1 region of a transgenic mouse that expressed a lacZ indicator gene controlled by the NR2C promoter region. Note that lacZ expression, while sparsely scattered throughout the layers, is particularly dense in layer 4. B, Somatic morphology of lacZ-expressing neurons in layer 4 (left) is very similar to that of SpS neurons, as shown in a 1 μm Nissl-stained section from a layer 4 brain slice previously used in a physiological experiment (right). lacZ is not expressed in layer 5 pyramidal neurons (middle).